Prospectus Supplement No. 12                    Filed Pursuant to Rule 424(b)(3)
(To Prospectus Dated May 19, 2000)                    Registration No. 333-35412

                              Prospectus Supplement
                             Dated January 12, 2001

This prospectus relates to the resale by the holders of our:

      o     $450,000,000  principal amount of 5.0%  convertible  notes due 2010,
            and

      o     the shares of Class A common stock  issuable upon  conversion of the
            notes.

This prospectus  supplement  should be read in conjunction  with, and may not be
delivered or utilized  without,  the prospectus dated May 19, 2000. The terms of
the notes are set forth in the prospectus.

The   information   in  the  table   appearing   under  the   heading   "Selling
Securityholders"  on page 33 in the  prospectus is hereby  amended by adding the
information below with respect to selling  securityholders not previously listed
in  the  prospectus  or  in  any  amendments  or  supplements  thereto,  and  by
superceding the information with respect to selling  securityholders  previously
listed in the prospectus or in any  amendments or  supplements  thereto that are
listed below:



                                                                                  Number of Shares                     Percentage of
                                                             Principal Amount     of Class A Common     Number of         Class A
                                                            at Maturity of 5.0%    Stock Issuable    Shares of Class   Common Stock
                                                            Notes Beneficially   Upon Conversion of   A Common Stock  Outstanding as
                                                              Owned That May     the 5.0% Notes That    Benefically     of January
                Selling Securityholders                          be Sold             May be Sold          Owned**        11, 2001***
                -----------------------                     -------------------  ------------------- ---------------  --------------
                                                                                                                

Alpha U.S. Sub Fund VIII, LLC                                      $200,000               3,883             --               *
AXP Utilities Income Fund, Inc.                                          $0                   0             --               *
Chrysler Corporation Master Retirement Trust                     $8,785,000             170,582             --               *
Credit Suisse First Boston Corporation (1)                               $0                   0             --               *
Deephaven Domestic Convertible Trading Ltd.                      $2,750,000              53,398             --               *
Delta Airlines Master Trust                                      $3,895,000              75,631             --               *
Goldman Sachs & Co. (1)                                                  $0                   0             --               *
McMahan Securities Co. L.P.                                              $0                   0             --               *
Minnesota Power, Inc.                                                    $0                   0             --               *
Motion Picture Industry Health Plan - Active Member Fund         $1,050,000              20,388             --               *
Motion Picture Industry Health Plan - Retiree Member Fund          $525,000              10,194             --               *
OCM Convertible Trust                                            $4,070,000              79,029             --               *
Partner Reinsurance Company, Ltd.                                $2,105,000              40,873             --               *
Sage Capital                                                     $1,730,000              33,592             --               *
State Employees' Retirement Fund of the State of Delaware        $4,605,000              89,417             --               *
State of Connecticut Combined Investment Funds                  $11,120,000             215,922             --               *
TCW Group, Inc.                                                          $0                   0             --               *
Vanguard Convertible Securities Fund, Inc.                      $10,350,000             200,970             --               *
                                                                ===========             =======           ====            ====
TOTAL                                                           $51,185,000             993,879             --               *


* Less than 1%.
** In addition to the shares issuable upon conversion of the notes.
*** Includes shares issuable upon conversion of the notes and additional  shares
beneficially owned as of January 11, 2000.

(1) Entity shown in the table,  or an  affiliate  of the entity,  was one of the
initial  purchasers  of these notes  and/or other notes of the Company that were
sold in a private  placement.  The initial  purchasers  acquired such notes at a
discount.  In  addition,  some  of  these  entities  or  their  affiliates  have
participated  in other  offerings  of  securities  by the  Company  and/or  have
performed other banking services for which they have received fees.