UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

Mitel Networks Corporation

(Name of Issuer)

Common Shares

(Title of Class of Securities)

60671Q104

(CUSIP Number)

Francisco Partners GP II, L.P.

One Letterman Drive

Building C, Suite 410

San Francisco, California 94129

Attention: Benjamin Ball

Telephone: (415) 418-2900

with a copy to:

Michael J. Kennedy, Esq.

Shearman & Sterling LLP

Four Embarcadero Center, Suite 3800

San Francisco, California 94111

Telephone: (415) 616-1100

(Name, Address and Telephone Number of Persons Authorized to Receive Notices and Communications)

November 10, 2013

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Exchange Act”) or otherwise subject to the liabilities of that section of the Exchange Act but shall be subject to all other provisions of the Exchange Act (however, see the Notes).

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This Amendment No. 3 to Schedule 13D is being filed jointly by the following (each a “Reporting Person” and collectively, the “Reporting Persons”): (1) Arsenal Holdco I, S.a.r.l., a Luxembourg societe a responsabilite limitee (“Arsenal I”), (2) Arsenal Holdco II, S.a.r.l., a Luxembourg societe a responsabilite limitee (“Arsenal II”), (3) Francisco Partners II (Cayman), L.P., a Cayman exempted limited partnership (“FP II Cayman”), (4) Francisco Partners GP II (Cayman), L.P., a Cayman exempted limited partnership (“FP GP II Cayman”), (5) Francisco Partners GP II Management (Cayman) Limited, a Cayman exempted company (“FP Management Cayman”), (6) Francisco Partners Parallel Fund II, L.P., a Delaware limited partnership (“FP Parallel Fund”), (7) Francisco Partners GP II, L.P., a Delaware limited partnership (“FP GP II”), and (8) Francisco Partners GP II Management, LLC, a Delaware limited liability company (“FP Management”), to supplement and amend the Schedule 13D filed on behalf of the Reporting Persons. Each item below amends and supplements the information disclosed under the corresponding item of Schedule 13D. Capitalized terms defined in the Schedule 13D are used herein with their defined meaning.

The response set forth in Item 5 of the Schedule 13D is hereby amended and supplemented by the following:

The following disclosure assumes there are 53,784,184 Common Shares outstanding, which the Company represented in its latest 10-Q filed on August 29, 2013 and representing the number of Common Shares outstanding as of August 27, 2013. All calculations of beneficial ownership and of the number of shares issuable upon the conversion or exercise of any securities are made as of November 17, 2013.

As of the date hereof, Arsenal I is deemed to beneficially own an aggregate of 14,570,738 Common Shares, which, based on calculations made in accordance with Rule 13d-3 of the Exchange Act, would constitute approximately 27.1% of the outstanding Common Shares, which includes 14,508,268 Common Shares owned by Arsenal I and 62,470 Common Shares owned by FP II Cayman. As of the date hereof, Arsenal II is deemed to beneficially own an aggregate of 5,590,136 Common Shares, which, based on calculations made in accordance with Rule 13d-3 of the Exchange Act, would constitute approximately 10.4% of the outstanding Common Shares, which includes 5,589,278 Common Shares owned by Arsenal II and 858 Common Shares owned by FP Parallel Fund.

Pursuant to Rule 13d-5 of the Exchange Act, by reason of the relationships described herein and in the Schedule 13D, Arsenal I and Arsenal II may be deemed to share beneficial ownership of the aggregate of 20,160,874 Common Shares, which, based on calculations made in accordance with Rule 13d-3 of the Exchange Act, would constitute approximately 37.5% of the outstanding Common Shares. The filing of this Statement shall not be construed as an admission that Arsenal I and Arsenal II beneficially own those shares held by the other Reporting Person, and Arsenal I and Arsenal II hereby disclaim such beneficial ownership.

The above does not include an additional 346,876 Common Shares issuable upon exercise of director options issued to director affiliates of Arsenal I and Arsenal II. This Statement shall not be construed as an admission that Arsenal I and Arsenal II beneficially own those shares held by the other Reporting Person, and Arsenal I and Arsenal II hereby disclaim such beneficial ownership.

FP II Cayman owns 62,470 Common Shares, additionally FP II Cayman, as the sole member of Arsenal I, FP GP II Cayman, as the general partner of FP II Cayman, and FP Management Cayman, as the general partner of FP GP II Cayman, may also be deemed to share voting and dispositive power of the Common Shares beneficially owned by Arsenal I. Except for the Common Shares held directly by FP II Cayman and to the extent of its interests as sole member of Arsenal I, FP II Cayman expressly disclaims such beneficial ownership. Except to the extent of its interests as general partner in FP II Cayman, FP GP II Cayman expressly disclaims such beneficial ownership. Except to the extent of its interest as general partner in FP GP II Cayman, FP Management Cayman expressly disclaims such beneficial ownership.

FP Parallel Fund owns 858 Common Shares, additionally, FP Parallel Fund, as the controlling member of Arsenal II, FP GP II, as the general partner of FP Parallel Fund, and FP Management, as the general partner of FP GP II, may also be deemed to share voting and dispositive power of the Common Shares beneficially owned by

Arsenal II. Except for the Common Shares held directly by FP Parallel Fund and to the extent of its interests as the controlling member of Arsenal II, FP Parallel Fund expressly disclaims such beneficial ownership. Except to the extent of its interests as general partner in FP Parallel Fund, FP GP II expressly disclaims such beneficial ownership. Except to the extent of its interest as general partner in FP GP, FP Management expressly disclaims such beneficial ownership.

Pursuant to Rule 13d-5 of the Exchange Act, by reason of the relationships described herein and in the Schedule 13D, Messrs. Ball, Deb, Stanton, Garfinkel, Geeslin, Golob, Perlman, Kowal, Shah, and Gallinelli and Ms. Silva may also be deemed to share voting and dispositive power of the Common Shares beneficially owned by Arsenal I and Arsenal II. Each of these individuals expressly disclaims such beneficial ownership.

The filing of this Amendment to Schedule 13D shall not be construed as an admission that any of the Reporting Persons share beneficial ownership for purposes of Section 13(d) of the Exchange Act.

The response set forth in Item 6 of the Schedule 13D is hereby amended and supplemented by the following:

Voting Agreement

In connection with the Arrangement Agreement entered into on November 10, 2013, by and between Aastra Technologies Limited (“Aastra”) and the Company (the “Arrangement Agreement”), Aastra, the Company, Arsenal I and Arsenal II have entered into a Voting Support Agreement, as of November 10, 2013 (the “Voting Agreement”). Pursuant to the Voting Agreement, Arsenal I and Arsenal II agreed in their capacity as Mitel stockholders to: (i) vote all of the Subject Shares (as defined in the Voting Agreement) held by them in favor of the approval, consent, ratification and adoption of the Arrangement Agreement and the transactions contemplated by the Arrangement Agreement, subject to certain customary exceptions; and (ii) not to sell, transfer, gift, assign, convey, pledge, hypothecate, encumber, option, grant a security interest in or otherwise dispose of any right or interest in any Subject Shares, subject to certain exceptions. Additionally, Arsenal I and Arsenal II have agreed for a period of 180 days following the effective date of the consummation of the transactions contemplated by the Arrangement Agreement not to sell, hypothecate, pledge, grant any option to purchase or otherwise dispose of or agree to dispose of, directly or indirectly, 18,144,786 Subject Shares, subject to certain exceptions. The Reporting Persons have not received any monetary consideration as a result of Arsenal I or Arsenal II entering into the Voting Agreement and have not purchased any securities of Mitel in connection with the transactions described herein.

The foregoing descriptions of the Voting Agreement does not purport to be complete and is qualified in its entirety by reference to the Voting Agreement, a copy of which is attached hereto as Exhibit A, and which is incorporated herein by reference. This Amendment No.3 does not purport to amend, qualify or in any way modify the Voting Agreement.

SIGNATURE

After reasonable inquiry and to the best of each of the undersigned’s knowledge and belief, the undersigned certifies that the information set forth in this statement is true, complete and correct.

Dated: November 17, 2013

Exhibit 99.1

VOTING SUPPORT AGREEMENT

THIS AGREEMENT is made as of November 10, 2013

BETWEEN:

AASTRA TECHNOLOGIES LIMITED, a corporation existing under the laws of Canada (“Aastra”)

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MITEL NETWORKS CORPORATION, a corporation existing under the laws of Canada (“Mitel”)

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ARSENAL HOLDCO I, S.A.R.L., a company existing under the laws of Luxembourg ( “Arsenal I”)

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ARSENAL HOLDCO II, S.A.R.L., a company existing under the laws of Luxembourg ( “Arsenal II” and together with Arsenal I, the “Securityholders” and each, a “Securityholder”)

RECITALS:

NOW THEREFORE, in consideration of the mutual covenants in this Agreement and for other good and valuable consideration (the receipt and sufficiency of which are hereby acknowledged) the parties hereto agree as follows:

ARTICLE 1

INTERPRETATION

Capitalized terms used herein and not otherwise defined have the meanings ascribed thereto in the Arrangement Agreement. In this Agreement, including the recitals:

“Arrangement Agreement” means the arrangement agreement dated as of the date hereof between Aastra and Mitel, as the same may be amended in accordance with its terms;

“Business Day” means any day, other than a Saturday, a Sunday or a statutory or civic holiday in Toronto, Ontario;

“Contracts” means contracts, licences, leases, agreements, obligations, promises, undertakings, understandings, arrangements, documents, commitments, entitlements or engagements to which a Securityholder is a party or by which it is bound or under which a Securityholder has, or will have, any liability or contingent liability (in each case, whether written or oral, express or implied), and includes any quotations, orders, proposals or tenders which remain open for acceptance and warranties and guarantees;

“Convertible Securities” has the meaning ascribed thereto in Section 2.1(b);

“Expiry Time” has the meaning ascribed thereto in Section 3.1(a);

“Locked-Up Securities” has the meaning ascribed thereto in Section 3.1(a);

“Notice” has the meaning ascribed thereto in Section 4.7;

“Subject Shares” means all Mitel Shares beneficially owned or controlled, directly or indirectly, by the Securityholders and their Affiliates as at the date hereof, and shall further include any Mitel Shares acquired or over which control is acquired by the Securityholders after the date hereof; and

“Transfer” has the meaning ascribed thereto in Section 3.1(a).

In this Agreement, words importing the singular number include the plural and vice versa and words importing gender include the masculine, feminine and neuter genders.

Unless otherwise expressly stated, all references to currency herein shall be deemed to be references to Canadian currency.

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The division of this Agreement into Articles, Sections and Schedules and the insertion of the recitals and headings are for convenience of reference only and shall not affect the construction or interpretation of this Agreement and, unless otherwise stated, all references in this Agreement or in the Schedules hereto to Articles, Sections and Schedules refer to Articles, Sections and Schedules of and to this Agreement or of the Schedules in which such reference is made, as applicable.

In the event that any date on which any action is required to be taken hereunder by any of the parties is not a Business Day, such action shall be required to be taken on the next succeeding day which is a Business Day.

This Agreement shall be governed, including as to validity, interpretation and effect, by the laws of the Province of Ontario and the laws of Canada applicable therein, and shall be construed and treated in all respects as an Ontario contract. Each of the parties hereby irrevocably attorns to the non-exclusive jurisdiction of the Courts of the Province of Ontario in respect of all matters arising under and in relation to this Agreement.

The Schedules attached hereto and described below shall, for all purposes hereof, form an integral part of this Agreement.

Schedule A – Subject Shares

Schedule B – Form of Written Consent

ARTICLE 2

REPRESENTATIONS AND WARRANTIES

The Securityholders, jointly and severally, represent and warrant to Aastra and Mitel (and acknowledge that Aastra and Mitel are relying on these representations and warranties in entering into the transactions contemplated hereby and by the Arrangement Agreement) the matters set out below:

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Aastra represents and warrants to the Securityholders (and acknowledges that the Securityholders are relying on these representations and warranties in completing the transactions contemplated hereby and by the Arrangement Agreement) that Aastra is a corporation duly incorporated and validly existing under the laws of the Canada and has all necessary corporate power, authority and capacity to enter into this Agreement and to carry out its obligations under this Agreement. The execution and delivery of this Agreement and the consummation of the transactions contemplated by this Agreement have been duly authorized by all necessary corporate action on the part of Aastra. This Agreement constitutes a valid and binding obligation of Aastra enforceable against it in accordance with its terms, subject to bankruptcy, insolvency and other Laws affecting the enforcement of creditors’ rights generally and subject to the qualification that equitable remedies may only be granted in the discretion of a court of competent jurisdiction.

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Mitel represents and warrants to the Securityholders (and acknowledges that the Securityholders are relying on these representations and warranties in completing the transactions contemplated hereby and by the Arrangement Agreement) that Mitel is a corporation duly incorporated and validly existing under the laws of the Canada and has all necessary corporate power, authority and capacity to enter into this Agreement and to carry out its obligations under this Agreement. The execution and delivery of this Agreement and the consummation of the transactions contemplated by this Agreement have been duly authorized by all necessary corporate action on the part of Mitel. This Agreement constitutes a valid and binding obligation of Mitel enforceable against it in accordance with its terms, subject to bankruptcy, insolvency and other Laws affecting the enforcement of creditors’ rights generally and subject to the qualification that equitable remedies may only be granted in the discretion of a court of competent jurisdiction.

ARTICLE 3

COVENANTS

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In connection with the foregoing, subject to this Section 3.1(b), the Securityholders hereby irrevocably and unconditionally agree, upon the request of Mitel or Aastra, to (A) deposit an irrevocable proxy, duly completed and executed in respect of all of the Subject Shares at least 10 days prior to any meeting of the holders of Mitel Shares to consider any of the matters identified in paragraph (i) of this Section 3.1(b), voting all such Subject Shares in favour of the applicable matter(s), and (B) deliver to Mitel or Aastra, as applicable, an irrevocable written consent, duly completed and executed in respect of all of the Subject Shares within 10 days of any request by Mitel or Aastra to provide such written consent in support of any of the matters identified in paragraph (i) of this Section 3.1(b) (including, without limitation, a consent addressed to the TSX and NASDAQ confirming that such holder is in favour of the Arrangement and the issuance of the Consideration Shares and Option Shares, which consent shall be substantially in the form attached as Schedule B hereto, with such changes as may be required by the TSX or NASDAQ). The Securityholders hereby irrevocably and unconditionally agree that neither it nor any person on its behalf will take any action to withdraw, amend or invalidate any proxy or consent deposited by the Securityholders pursuant to this Agreement notwithstanding any statutory or other rights which the Securityholders might have unless this Agreement is terminated in accordance with Section 4.1.

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The Securityholders hereby authorize Mitel and its transfer agent, during the Lock-Up Period, to decline the transfer of, or to note stop transfer restrictions on the stock register and other records relating to the Locked-Up Securities of which the Securityholders are the registered holder, and, with respect to Locked-Up Securities of which the Securityholders are the beneficial owner but not the registered holder, the Securityholders hereby agree to cause such registered holder to authorize Mitel and its transfer agent, during the Lock-Up Period, to decline the transfer of or to note stop transfer restrictions on the stock register and other records relating to such Locked-Up Securities. Notwithstanding the foregoing, if any other party is released, in full or in part, from the lock-up restrictions of any similar voting support agreement related to the transactions contemplated by the Arrangement Agreement, then the Securityholders shall be released in the same manner from the restrictions of this Agreement (i.e., in the case where shares of such individual or entity are released from lock-up restrictions, the same percentage of shares held by the Securityholders shall be released from the restrictions of this Agreement on the same terms).

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ARTICLE 4

GENERAL

This Agreement shall terminate and be of no further force or effect only upon the earliest of:

Any date, time or period referred to in this Agreement shall be of the essence, except to the extent to which the Securityholders, Aastra and Mitel agree in writing to vary any date, time or period, in which event the varied date, time or period shall be of the essence.

The parties agree that irreparable harm will occur for which money damages will not be an adequate remedy at Law in the event that any of the provisions of this Agreement are not performed by the Securityholders in accordance with their terms or are otherwise breached. It is accordingly agreed that Aastra and Mitel shall be entitled to an injunction or injunctions and other equitable relief to prevent breaches or threatened breaches of the provisions of this Agreement by the Securityholders and shall be entitled to obtain specific performance by the

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Securityholders of any such provisions. The Securityholders hereby agree not to seek the posting of any security bond or other assurance in respect of such injunctive or other equitable relief. Such remedies will not be the exclusive remedies for any breach of this Agreement and will be in addition to all other remedies available at Law or equity.

Aastra and Mitel acknowledge that the Securityholders are not making any agreement or understanding herein in any capacity other than in their capacity as securityholders of Mitel. Nothing herein shall restrict the Securityholders from taking in good faith any actions necessary to discharge such securityholders’ fiduciary duties as a director or officer of Mitel.

Each party hereto agrees and confirms that any provision of this Agreement may be amended or waived if, and only if, such amendment or waiver is in writing and signed, in the case of an amendment, by the Securityholders, Aastra and Mitel or in the case of a waiver, by the party against whom the waiver is to be effective and no failure or delay by any party in exercising any right, power or privilege hereunder shall operate as a waiver thereof nor shall any single or partial exercise thereof preclude any other or further exercise.

This Agreement constitutes the entire agreement among the parties with respect to the subject matter hereof and supersedes all prior agreements and understandings among the parties with respect thereto.

Any notice, consent or approval required or permitted to be given in connection with this Agreement (in this Section referred to as a “Notice”) shall be in writing and shall be sufficiently given if delivered (whether in person, by courier service or other personal method of delivery), or if transmitted by facsimile or e-mail:

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Any Notice delivered or transmitted to a party as provided above shall be deemed to have been given and received on the day it is delivered or transmitted, provided that it is delivered or transmitted on a Business Day prior to 5:00 p.m. local time in the place of delivery or receipt. If the Notice is delivered or transmitted after 5:00 p.m. local time or if the day is not a Business Day, then the Notice shall be deemed to have been given and received on the next Business Day.

Either party hereto may, from time to time, change its address by giving Notice to the other parties in accordance with the provisions of this Section 4.7.

If, in any jurisdiction, any provision of this Agreement or its application to any party or circumstance is restricted, prohibited or unenforceable, such provision shall, as to such jurisdiction, be ineffective only to the extent of such restriction, prohibition or unenforceability without invalidating the remaining provisions of this Agreement and without affecting the validity or enforceability of such provision in any other jurisdiction or without affecting its application to other parties or circumstances. Upon such determination that any term or other provision is invalid, illegal or incapable of being enforced, the parties hereto shall negotiate in good faith to modify this Agreement so as to effect the original intent of the parties as closely as possible in a mutually acceptable manner in order that the terms of this Agreement remain as originally contemplated to the fullest extent possible.

The provisions of this Agreement shall be binding upon and enure to the benefit of the parties hereto and their respective successors, permitted assigns and legal personal representatives, provided that no party may assign, delegate or otherwise transfer any of its rights, interests or obligations under this Agreement without the prior written consent of the other parties hereto, except that Aastra may assign, delegate or otherwise transfer any of its rights, interests or obligations under this Agreement to an Affiliate without reducing its own obligations hereunder without the consent of the Securityholders.

Each party shall pay all costs and expenses (including the fees and disbursements of legal counsel and other advisers) it incurs in connection with the negotiation, preparation and execution of this Agreement and the transactions contemplated by this Agreement.

The Securityholders acknowledge that they have been afforded the opportunity to obtain independent legal advice and confirm by the execution of this Agreement that they have either done so or waived its right to do so in connection with the entering into of this Agreement.

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The parties hereto shall, with reasonable diligence, do all things and provide all such reasonable assurances as may be required to consummate the transactions contemplated by this Agreement, and each party shall provide such further documents or instruments required by the other party as may be reasonably necessary or desirable to effect the purpose of this Agreement and carry out its provisions, whether before or after the Effective Time.

This Agreement may be executed in one or more counterparts, each of which shall be deemed to be an original but all of which together shall constitute one and the same instrument. The Parties shall be entitled to rely upon delivery of an executed facsimile or similar executed electronic copy of this Agreement, and such facsimile or similar executed electronic copy shall be legally effective to create a valid and binding agreement between the Parties.

[Signature page follows.]

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IN WITNESS OF WHICH the parties have executed this Agreement.

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SCHEDULE “A”

A-1

SCHEDULE “B”

FORM OF WRITTEN CONSENT

(see attached)