Exhibit 4.16

                               AMENDING AGREEMENT

      THIS AMENDING AGREEMENT made effective as of April 23, 2004.

BETWEEN:

            MITEL NETWORKS  CORPORATION,  a corporation  incorporated  under the
            laws of Canada,

            (the "Corporation")

                                     - and -

            ZARLINK  SEMICONDUCTOR  INC., a corporation  incorporated  under the
            laws of Canada,

            ("Zarlink")

RECITALS:

A.    The Corporation  and Zarlink are party to a supply  agreement (the "Supply
      Agreement") dated February 16, 2001, as amended.

B.    Clause  21(A)(ii)  of  the  Supply  Agreement  provides  that  the  Supply
      Agreement  may  be   terminated   by  one  party  in  certain   prescribed
      circumstances  where a change occurs in the  constitution or circumstances
      of the other party, as more particularly set forth therein.

C.    In connection with a proposed financing  transaction pursuant to which new
      investors of the Corporation  will subscribe for certain  preferred shares
      in the capital of the  Corporation  (the  "Financing"),  and in connection
      with which the  Corporation  proposes to effect certain  amendments to its
      constating  documents  as set out in the  management  proxy  circular  and
      related  shareholder  meeting materials of the Corporation dated March 26,
      2004  (the  "Meeting  Materials"),  a copy of which has been  provided  to
      Zarlink,  the  Corporation  and  Zarlink now desire to provide for certain
      acknowledgements and amendments with respect to the Supply Agreement.

THEREFORE THE PARTIES AGREE AS FOLLOWS:

1.    Acknowledgement

            Zarlink  hereby  acknowledges  and  agrees,  for  purposes of clause
            21(A)(ii) of the Supply Agreement, that:

      (a)   the  Corporation  proposes  to  effect  certain  amendments  to  its
            constating documents in connection with the Financing, substantially
            as described in the Meeting Materials;

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      (b)   such proposed amendments are not materially detrimental to Zarlink's
            interests and will not impede the Corporation's ability to carry out
            its obligations under the Supply Agreement; and

      (c)   such proposed  amendments  shall not give rise to a right of Zarlink
            to terminate the Supply Agreement.

2.    Amendment

      The Supply  Agreement is hereby  amended as follows:  Clause  21(A)(ii) is
      hereby deleted in its entirety.

3.    Binding Amendment

      This   Amending   Agreement,   being  duly   executed  by  an   authorized
      representative of each of the Corporation and Zarlink, constitutes a valid
      and binding  amendment to the Supply  Agreement in accordance with section
      32 thereof.

4.    General

      (a)   This Amending  Agreement  and the Supply  Agreement  constitute  the
            entire  agreement  between the parties  with  respect to the subject
            matter  hereof  and  thereof  and  cancel  and  supersede  any prior
            understandings  and  agreements  between the parties with respect to
            such  subject  matter.  There  are no  representations,  warranties,
            terms, conditions,  undertakings or collateral agreements,  express,
            implied or statutory, between the parties to this Amending Agreement
            other  than  those  expressly  set forth  herein  and in the  Supply
            Agreement.

      (b)   Except as provided in this Amending Agreement, in all other respects
            the Supply Agreement remains in full force and effect.

      (c)   Each of the parties  hereto shall with  reasonable  diligence do all
            such  things and provide all such  reasonable  assurances  as may be
            required  to  consummate  the  transactions   contemplated  by  this
            Amending  Agreement,  and each  party  shall  provide  such  further
            documents  or  instruments  required  by any  other  party as may be
            reasonably  necessary  or  desirable  to effect the  purpose of this
            Amending Agreement and carry out its provisions.

      (d)   This Amending  Agreement  shall be construed in accordance  with the
            laws of the  Province of Ontario  and the laws of Canada  applicable
            therein and shall be treated in all respects as an Ontario contract.

      (e)   This  Amending  Agreement  may be executed in several  counterparts,
            each of which,  when executed by a party hereto,  shall be deemed to
            be an original and such counterparts  shall together  constitute one
            and the same instrument.

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IN WITNESS OF WHICH the parties have executed this Amending Agreement.

                                      MITEL NETWORKS CORPORATION

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

                                      ZARLINK SEMICONDUCTOR INC.

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