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                                                                   EXHIBIT 10.25

                          LETTER AMENDMENT NO. 1 TO THE
                           FIVE YEAR CREDIT AGREEMENT

                                                     Dated as of August 10, 2001

To the banks, financial institutions
  and other institutional lenders
  (collectively, the "Lenders") parties
  to the Credit Agreement referred to
  below and to Citibank, N.A., as agent
  (the "Agent") for the Lenders

Ladies and Gentlemen:

                  We refer to the Five Year Competitive Advance and Revolving
Credit Facility Agreement dated as of September 25, 2000 (the "CREDIT
AGREEMENT") among the undersigned, Lucent Technologies Inc. and you. Capitalized
terms not otherwise defined in this Letter Amendment have the same meanings as
specified in the Credit Agreement. Pursuant to the terms of the Credit
Agreement, Lucent Technologies Inc. has ceased to be a party to the Credit
Agreement.

                  It is hereby agreed by you and us that the Credit Agreement
is, effective as of the date of this Letter Amendment, hereby amended as
follows:

                  The definition of "CONSOLIDATED EBIT" in Section 1.01 is
amended in full to read as follows:

                           "Consolidated EBIT" shall mean, for any period, net
         income (or net loss) PLUS the sum of (a) consolidated interest expense
         and (b) consolidated income tax expense, in each case determined in
         accordance with GAAP for such period, EXCLUDING, up to $950,000,000 of
         charges in connection with the business restructuring plan during such
         period to be taken no later than the fourth quarter of fiscal year 2001
         of the Borrower, up to $300,000,000 of start-up costs associated with
         the establishment of the Borrower as a separate business entity
         incurred during the period to be taken through the fourth quarter of
         fiscal year 2001 of the Borrower and up to $450,000,000 of non-cash
         business restructuring charges during such period to be taken no later
         than the fourth quarter of fiscal year 2001 of the Borrower.

                  This Letter Amendment shall become effective as of the date
first above written when, and only when, the Agent shall have received
counterparts of this Letter Amendment executed by the undersigned and the
Required Lenders or, as to any of the Lenders, advice satisfactory to the Agent
that such Lender has executed this Letter Amendment. This Letter Amendment is
subject to the provisions of Section 8.08 of the Credit Agreement.

                  On and after the effectiveness of this Letter Amendment, each
reference in the

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Credit Agreement to "this Agreement", "hereunder", "hereof" or words of like
import referring to the Credit Agreement, and each reference in the Notes to
"the Credit Agreement", "thereunder", "thereof" or words of like import
referring to the Credit Agreement, shall mean and be a reference to the Credit
Agreement, as amended by this Letter Amendment.

                  The Credit Agreement and the Notes, as specifically amended by
this Letter Amendment, are and shall continue to be in full force and effect and
are hereby in all respects ratified and confirmed. The execution, delivery and
effectiveness of this Letter Amendment shall not, except as expressly provided
herein, operate as a waiver of any right, power or remedy of any Lender or the
Agent under the Credit Agreement, nor constitute a waiver of any provision of
the Credit Agreement.

                  If you agree to the terms and provisions hereof, please
evidence such agreement by executing and returning at least two counterparts of
this Letter Amendment to Susan L. Hobart, Shearman & Sterling, 599 Lexington
Avenue, New York, New York 10022.

                  This Letter Amendment may be executed in any number of
counterparts and by different parties hereto in separate counterparts, each of
which when so executed shall be deemed to be an original and all of which taken
together shall constitute one and the same agreement. Delivery of an executed
counterpart of a signature page to this Letter Amendment by telecopier shall be
effective as delivery of a manually executed counterpart of this Letter
Amendment.

                  This Letter Amendment shall be governed by, and construed in
accordance with, the laws of the State of New York.

                                      Very truly yours,
                                      AVAYA INC.

                                      By /s/ Rhonda Seegal
                                         ---------------------------------------
                                         Name: Rhonda Seegal
                                         Title: Treasurer

                                      Agreed as of the date first above written:

                                      CITIBANK, N.A.,
                                           as Agent and as Lender

                                      By /s/ Carolyn A. Kee
                                         ---------------------------------------
                                         Name: Carolyn A. Kee
                                         Title: Vice President

                                      BANK ONE, NA (Main Office Chicago)

                                      By /s/ Lori J. Thomas
                                         ---------------------------------------
                                         Name: Lori J. Thomas
                                         Title: Vice President
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                                      THE CHASE MANHATTAN BANK

                                      By /s/ Dennis R. Wilczek
                                         ---------------------------------------
                                         Name: Dennis R. Wilczek
                                         Title: Vice President

                                      DEUTSCHE BANK AG NEW YORK BRANCH

                                      By /s/ Peter Eschmann
                                         ---------------------------------------
                                         Name: Peter Eschmann
                                         Title: Associate

                                      By /s/ Virginia Mahler
                                         ---------------------------------------
                                         Name: Virginia Mahler
                                         Title: Vice President

                                      COMMERZBANK AG

                                      By
                                         ---------------------------------------
                                         Name:
                                         Title:

                                      THE BANK OF NEW YORK

                                      By
                                         ---------------------------------------
                                         Name:
                                         Title:

                                      THE BANK OF TOKYO -
                                      MITSUBISHI LTD., NEW YORK BRANCH

                                      By
                                         ---------------------------------------
                                         Name:
                                         Title:

                                      CREDIT SUISSE FIRST BOSTON

                                      By /s/ Jeffrey Bernstein
                                         ---------------------------------------
                                         Name: Jeffrey Bernstein
                                         Title: Vice President

                                      By /s/ Robert Hetu
                                         ---------------------------------------
                                         Name: Robert Hetu
                                         Title: Director

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                                      HSBC BANK USA

                                      By
                                         ---------------------------------------
                                         Name:
                                         Title:

                                      THE NORTHERN TRUST COMPANY

                                      By
                                         ---------------------------------------
                                         Name:
                                         Title:

                                      THE SUMITOMO BANK, LIMITED

                                      By
                                         ---------------------------------------
                                         Name:
                                         Title:

                                      WESTDEUTSCHE LANDESBANK
                                      GIROZENTRALE, NEW YORK BRANCH

                                      By /s/ Barry S. Wadler
                                         ---------------------------------------
                                         Name: Barry S. Wadler
                                         Title: Associate Director

                                      By /s/ Lisa M. Walker
                                         ---------------------------------------
                                         Name: Lisa M. Walker
                                         Title: Associate Director<Page>

                                                                   Exhibit 4.4

                      SECOND AMENDMENT TO RIGHTS AGREEMENT
                      ------------------------------------

     SECOND AMENDMENT TO RIGHTS AGREEMENT (the "Second Amendment") dated as
of June 30, 2001, to the Rights Agreement (the "Rights Agreement") dated as
of July 1, 1991, as amended by the First Amendment to Rights Agreement (the
"First Amendment") dated as of February 21, 1997, by and between VERTEX
PHARMACEUTICALS INCORPORATED, a Massachusetts corporation (the "Company"),
and FLEET NATIONAL BANK (f/k/a BANK BOSTON, N.A., also f/k/a THE FIRST
NATIONAL BANK OF BOSTON), as Rights Agent (the "Rights Agent").

     The Company and the Rights Agent have heretofore executed and entered
into the Rights Agreement and the First Amendment thereto. Pursuant to
SECTION 27 of the Rights Agreement, the Company and the Rights Agent may from
time to time supplement or amend the Rights Agreement in accordance with the
provisions of SECTION 27 thereof. All acts and things necessary to make this
Second Amendment a valid agreement, enforceable according to its terms, have
been done and performed, and the execution and delivery of this Second
Amendment by the Company and the Rights Agent have been in all respects duly
authorized by the Company and the Rights Agent.

     In consideration of the foregoing and the mutual agreements set forth
herein, the parties hereto agree as follows:

     1.  SECTION 27(a)(i) of the Rights Agreement is hereby modified and
amended by deleting the date June 30, 2001, and substituting therefore the
date June 30, 2011.

     2.  In each instance that the term "Final Expiration Date" is used in
the Rights Agreement, the use of such term shall mean the date June 30, 2011.

     3.  If any term, provision, covenant or restriction of this Second
Amendment is held by a court of competent jurisdiction or other authority to
be invalid, void or unenforceable, the remainder of the terms, provisions,
covenants and restrictions of this Second Amendment, the Rights Agreement and
the First Amendment shall remain in full force and effect and shall in no way
be affected, impaired or invalidated.

     4.  This Second Amendment shall be deemed to be a contract made under
the laws of the Commonwealth of Massachusetts and for all purposes shall be
governed by and construed in accordance with the laws of such Commonwealth
applicable to contracts to be made and performed entirely within such
Commonwealth.

     5.  This Second Amendment may be executed in any number of counterparts
and each of such counterparts shall for all purposes be deemed to be an
original, and all such counterparts shall together constitute but one and the
same instrument.

     6.  In all respects not inconsistent with the terms and provisions of
this Second Amendment, the Rights Agreement and the First Amendment thereto
are hereby ratified, adopted, approved and confirmed. In executing and
delivering this Second Amendment, the

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Rights Agent shall be entitled to all the privileges and immunities afforded
to the Rights Agent under the terms and conditions of the Rights Agreement
and the First Amendment.

     IN WITNESS WHEREOF, the parties hereto have caused this Second Amendment
to be duly executed and attested, all as of the date and year first above
written.

ATTEST:                                  VERTEX PHARMACEUTICALS
                                         INCORPORATED

By: /s/ Sarah P. Cecil                   By: /s/ Vicki L. Sato
   --------------------------------         ----------------------------------
   Name: Sarah P. Cecil                     Name: Vicki L. Sato, Ph.D.
        ---------------------------              -----------------------------
   Title: Corporate Counsel                 Title: President
         --------------------------               ----------------------------

ATTEST:                                  FLEET NATIONAL BANK
                                         f/k/a BANK BOSTON, N.A.
                                         also f/k/a THE FIRST NATIONAL BANK
                                         OF BOSTON

By: /s/ Jocelyn J. Turner               By: /s/ Carol Mulvey-Eori
   --------------------------------         ----------------------------------
   Name: Jocelyn J. Turner                  Name: Carol Mulvey-Eori
        ---------------------------              -----------------------------
   Title: Sr. Account Manager               Title: Managing Director
         --------------------------               ----------------------------

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