SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR 13(e)(1)

OF THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 4)

iBASIS, INC.

(Name of Subject Company (Issuer))

KPN B.V.

(Offeror)

KONINKLIJKE KPN N.V.

(Parent of Offeror)

CELTIC ICS INC.

(Wholly Owned Subsidiary of Offeror)

(Names of Filing Persons (identifying status as offeror, issuer or other person))

COMMON STOCK, $0.001 PAR VALUE

(Title of Class of Securities)

450732201

(CUSIP Number of Class of Securities)

Michel Hoekstra

Koninklijke KPN N.V.

Maanplein 55

2516 CK, The Hague, The Netherlands

+31 70 446 2093

(Name, address, and telephone numbers of person authorized to receive

notices and communications on behalf of filing persons)

Copies to:

Mark I. Greene, Esq.

Cravath, Swaine & Moore LLP

Worldwide Plaza

825 Eighth Avenue

New York, New York 10019

(212) 474-1000

CALCULATION OF FILING FEE

Transaction Valuation(1)	Amount of Filing Fee(2)
$48,216,244	$2,690

(1)           Estimated for purposes of calculating the filing fee only.  This amount is determined by multiplying 31,107,254 shares of common stock, par value $0.001 per share (the “Shares”), of iBasis, Inc. (the “Company”) by $1.55 per Share, which is the offer price.  Such number of Shares represents the 71,228,328 Shares issued and outstanding as of July 31, 2009, as reported in the Company’s Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2009, less the 40,121,074 Shares already owned by KPN B.V.

(2)           The filing fee was calculated in accordance with Rule 0-11 under the Securities Exchange Act of 1934 and Fee Rate Advisory #5 for fiscal year 2009, issued March 11, 2009, by multiplying the transaction value by .0000558.

ý Check the box if any part of the fee is offset as provided by Rule 0-11(a)(2) and identify the filing with which the offsetting fee was previously paid. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

Amount Previously Paid: $2,690	Filing Party: Koninklijke KPN N.V., KPNB.V., Celtic ICS Inc.
Form or Registration No.: Schedule TO-T	Date Filed: July 28, 2009

¨ Check the box if the filing relates solely to preliminary communications made before the commencement of a tender offer.

Check the appropriate boxes below to designate any transactions to which the statement relates:

ý third-party tender offer subject to Rule 14d-1.

¨ issuer tender offer subject to Rule 13e-4.

ý going-private transaction subject to Rule 13e-3.

¨ amendment to Schedule 13D under Rule 13d-2.

Check the following box if the filing is a final amendment reporting the results of the tender offer. ¨

If applicable, check the appropriate box(es) below to designate the appropriate rule provision(s) relied upon:

¨      Rule 13e-4(i) (Cross-Border Issuer Tender Offer)

¨      Rule 14d-1(d) (Cross-Border Third-Party Tender Offer)

This Amendment No. 4 amends and supplements the Tender Offer Statement and Rule 13e-3 Transaction Statement filed under cover of Schedule TO with the Securities and Exchange Commission (the “SEC”) on July 28, 2009, as amended and supplemented from time to time (the “Schedule TO”), by (i) KPN B.V., a private limited liability company organized under the laws of The Netherlands (“Purchaser”) and a wholly owned subsidiary of Koninklijke KPN N.V., a public company incorporated under the laws of The Netherlands (“Parent”), (ii) Parent and (iii) Celtic ICS Inc., a Delaware corporation (“Merger Sub”) and a wholly owned subsidiary of Purchaser.  The Schedule TO relates to the offer (the “Offer”) by Purchaser to purchase all of the outstanding shares of common stock, par value $0.001 per share (the “Shares”), of iBasis, Inc., a Delaware corporation (the “Company”), that are not already owned by Purchaser at a purchase price of $1.55 per Share, net to the seller in cash, without interest and less any required withholding taxes, upon the terms and subject to the conditions set forth in the Offer to Purchase dated July 28, 2009 (together with any amendments and supplements thereto, the “Offer to Purchase”) and in the related Letter of Transmittal.

Items 1–13

As set forth in Amendment No. 3 to the Tender Offer Statement and Rule 13e-3 Transaction Statement filed August 21, 2009, the Expiration Date of the Offer is extended to 12:00 midnight, New York City time, on Friday, October 23, 2009 (which is the end of the day on October 23, 2009). As of the close of business on August 21, 2009, approximately 75,775 Shares have been tendered pursuant to the Offer.

SIGNATURES

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

KPN B.V.
by KONINKLIJKE KPN N.V., its sole director
By:	/s/ Michel Hoekstra
	Name:  Michel Hoekstra
	Title:    Corporate Legal Officer & Attorney-in-Fact
	Date:   August 21, 2009

KONINKLIJKE KPN N.V.
By:	/s/ Michel Hoekstra
	Name:  Michel Hoekstra
	Title:    Corporate Legal Officer & Attorney-in-Fact
	Date: August 21, 2009

CELTIC ICS INC.
By:	/s/ Michel Hoekstra
	Name:  Michel Hoekstra
	Title:   Secretary
	Date:   August 21, 2009

EXHIBIT INDEX

Exhibit No.	Description
(a)(1)(i)	Offer to Purchase dated July 28, 2009*
(a)(1)(ii)	Letter of Transmittal*
(a)(1)(iii)	Notice of Guaranteed Delivery*
(a)(1)(iv)	Letter to Brokers, Dealers, Commercial Banks, Trust Companies and Other Nominees*
(a)(1)(v)	Letter to Clients for Use by Brokers, Dealers, Commercial Banks, Trust Companies and Other Nominees*
(a)(1)(vi)	Summary Advertisement published in the Wall Street Journal on July 28, 2009*
(a)(1)(vii)	Letter dated July 12, 2009 to the board of directors of iBasis, Inc. (incorporated by reference to Exhibit D to the Schedule 13D amendment filed by Purchaser, Parent and Merger Sub on July 13, 2009)*
(a)(1)(viii)	Press release issued by Parent on July 13, 2009 (incorporated by reference to Exhibit E to the Schedule 13D amendment filed by Purchaser, Parent and Merger Sub on July 13, 2009)*
(a)(1)(ix)	Press release issued by Parent on August 6, 2009*
(a)(1)(x)	Press release issued by Parent on August 13, 2009*
(a)(1)(xi)	Press release issued by Parent on August 21, 2009*
(a)(5)(i)	Complaint filed by the Company on August 3, 2009 in action captioned as iBasis, Inc. v. Koninklijke KPN N.V., et al., Civil Action No. 4774-VCS, in the Court of Chancery of the State of Delaware (incorporated by reference to Exhibit (a)(9) to Amendment No. 2 to the Company’s Solicitation/Recommendation Statement on Schedule 14D-9 filed on August 4, 2009)*
(a)(5)(ii)	Answer and Counterclaims filed by Parent on August 13, 2009 in action captioned as iBasis, Inc. v. Koninklijke KPN N.V., et al., Civil Action No. 4774-VCS, in the Court of Chancery of the State of Delaware*
(a)(5)(iii)	Complaint filed by the Company on August 18, 2009 in action captioned as iBasis, Inc. v. Koninklijke KPN N.V., et al., No. 09 CV 7288, in the United States District Court for the Southern District of New York (incorporated by reference to Exhibit (a)(18) to Amendment No. 8 to the Company’s Solicitation/Recommendation Statement on Schedule 14D-9 filed on August 18, 2009)*
(b)	Euro 1,500,000,000 Syndicated Revolving Credit Agreement dated as of August 16, 2006 among Parent,  ABN AMRO Bank N.V., as facility agent, an original lender, a mandated lead arranger, euro swingline agent and dollar swingline agent, and the other financial institutions party thereto (incorporated by reference to Exhibit C to the Schedule 13D amendment filed by Purchaser, Parent and Merger Sub on July 13, 2009)*
(c)(i)	Presentation of Morgan Stanley dated July 8, 2009*
(c)(ii)	Presentation of Morgan Stanley dated June 3, 2009*
(d)(i)	Share Purchase and Sale Agreement dated as of June 21, 2006 between the Company and Purchaser (incorporated by reference to Exhibit 2 to the Schedule 13D amendment filed by Purchaser and Parent on October 11, 2007)*
(d)(ii)	Amendment No. 1 dated as of December 18, 2006 to Share Purchase and Sale Agreement between the Company and Purchaser (incorporated by reference to Exhibit 3 to the Schedule 13D amendment filed by Purchaser and Parent on October 11, 2007)*
(d)(iii)	Amendment No. 2 dated as of April 26, 2007 to Share Purchase and Sale Agreement between the Company  and Purchaser (incorporated by reference to Exhibit 4 to the Schedule 13D amendment filed by Purchaser and Parent on October 11, 2007)*
(d)(iv)	Amendment No. 3 dated as of August 1, 2007 to Share Purchase and Sale Agreement between the Company  and Purchaser (incorporated by reference to Exhibit 5 to the Schedule 13D amendment filed by Purchaser and Parent on October 11, 2007)*
(d)(v)	Registration Rights Agreement dated as of October 1, 2007 between the Company and Purchaser (incorporated by reference to Exhibit 4.01 to the Current Report on Form 8-K filed by the Company on October 5, 2007)*
(d)(vi)	Second Amended and Restated By-laws of the Company (incorporated by reference to Exhibit 3.01 to the Current Report on Form 8-K filed by the Company on October 5, 2007)*
(d)(vii)	Amendment No. 1 to the Second Amended and Restated By-laws of the Company (incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K filed by the Company on November 14, 2007)*
(f)	Section 262 of the General Corporation Law of the State of Delaware (included as Schedule II to the Offer to Purchase)*
(g)	None
(h)	None
(i)	Power of Attorney dated as of July 11, 2009*
* Previously filed