<PAGE>

                                                                   Exhibit 10.43


THIS AGREEMENT is made effective as of the ______ day of April, 2000.

BETWEEN:

                         WORLDWIDE FIBER HOLDINGS LTD.

                                 (the "Vendor")

                                                               OF THE FIRST PART

                                    - and -

                            URBANLINK HOLDINGS LTD.

                               (the "Purchaser")

                                                              OF THE SECOND PART


                              ROLL-OVER AGREEMENT


                                    (SHARES)

A.    The Vendor is the beneficial owner of the Shares.

B.    The Vendor wishes to sell and the Purchaser wishes to purchase all of the
Vendor's interest in the Shares as at the Effective Date for a total purchase
price equal to the aggregate of the fair market values of the Shares all on the
terms and conditions and subject to the representations and warranties set forth
in this Agreement.

      IN CONSIDERATION of the foregoing premises and of the mutual covenants
herein contained the parties agree as follows:

                      ARTICLE 1: INCORPORATION OF RECITALS
                         AND SCHEDULES AND DEFINITIONS

1.1   The Recitals and all Schedules annexed to this Agreement are expressly
incorporated into, and form an integral part of, this Agreement.

                             ARTICLE 2: DEFINITIONS

2.1   In this Agreement:

      (a)   "Act" means the Income Tax Act (Canada), as it read on the Effective
            Date;

<PAGE>

      (b)   "Adjusted Cost Base" means the aggregate cost base of the Shares
            determined by the Vendor in accordance with the Act, where "adjusted
            cost base" is as defined in subsection 248(1) of the Act;

      (c)   "Adjusted Elected Amount" means the amount that, for tax purposes,
            is to be the Vendor's "proceeds of disposition" and the Purchaser's
            "cost of acquisition" of the Shares, determined in accordance with
            section 6.3 to be a greater or lesser amount than the Elected
            Amount;

      (d)   "Articles" means the articles of incorporation of the Purchaser
            together with all amendments thereto as registered with the
            Registrar of Corporations for the Province of Alberta as at the
            Effective Date;

      (e)   "Closing Date" means the Effective Date;

      (f)   "Corporation" means 360networks inc.;

      (g)   "Effective Date" means the date of this Agreement;

      (h)   "Elected Amount" means the amount agreed upon by the parties in
            accordance with section 6.2 hereof which, for tax purposes, is to be
            the Vendor's "proceeds of disposition" and the Purchaser's "cost of
            acquisition" of the Shares;

      (i)   "Election" means the election made by the parties in prescribed form
            and within the time prescribed by subsection 85(6) of the Act that
            the rules set forth in subsection 85(1) of the Act shall apply to
            the purchase and sale of the Shares;

      (j)   "Fair Market Value" means the actual fair market value of the Shares
            determined as of the Effective Date by agreement of the Vendor and
            Purchaser;

      (k)   "Revised Adjusted Cost Base" means the adjusted cost base of the
            Shares determined in accordance with section 6.3 to be greater or
            lesser amounts than the Adjusted Cost Base;

      (l)   "Schedules" means any schedules which are attached to this
            Agreement;

      (m)   "Share Consideration" means the number, series and class of shares
            of the capital of the Purchaser having attached thereto only those
            rights, privileges, restrictions and limitations set forth in the
            Articles, which shares are more particularly described in Schedule
            "B";

      (n)   "Shares" means those shares in the capital of the Corporation owned
            by the Vendor, more particularly described in Schedule "A";


                                      -2-
<PAGE>

      (o)   "Stated Capital Addition" means the aggregate stated capital for the
            shares constituting the Share Consideration that is to be added to
            the stated capital account maintained for such shares, which shall
            be equal to the Elected Fair Market Value; and

      (p)   "Taxing Authority" means the Minister of National Revenue for Canada
            or any other competent authority having jurisdiction that is
            authorized by law to issue an assessment or reassessment in respect
            of the transactions herein contemplated.

                          ARTICLE 3: PURCHASE AND SALE

3.1   Subject to the terms and conditions, and based upon the representations
and warranties, contained in this Agreement, the Vendor shall sell, transfer and
assign and the Purchaser shall purchase as of the Effective Date all of the
Vendor's interest in the Shares.

                           ARTICLE 4: EFFECTIVE DATE

4.1   It is acknowledged and agreed to by the parties that the purchase and sale
of the Shares is deemed to have occurred as of the Effective Date.

                           ARTICLE 5: PURCHASE PRICE

5.1   The total purchase price for the Shares is equal to the Fair Market Value
and shall be paid and satisfied by the Purchaser issuing to the Vendor the Share
Consideration.

5.2   The Purchaser shall add the Stated Capital Addition to the stated capital
account being maintained for the class of shares of which the Share
Consideration forms a part.

5.3   The parties' determination of the Fair Market Value is final and binding
upon the parties.

                 ARTICLE 6: JOINT ELECTION AND ELECTED AMOUNTS

6.1   The Vendor and Purchaser shall jointly make the Election.

6.2   Subject to the provisions of this Agreement, for purposes of the Election
the parties agree that the Elected Amount shall be the Adjusted Cost Base of the
Shares, which determination is final and binding on the parties.

6.3   If:

      (a)   the Taxing Authority proposes to issue, or does issue, any
            assessment or reassessments that impose, or would impose, any
            liability for tax of any


                                      -3-
<PAGE>

            nature or kind on either of the parties on the basis that the
            adjusted cost base of the Shares is the Revised Adjusted Cost Base;

      (b)   the parties agree, or a Court or tribunal of competent jurisdiction
            finally adjudges, that the adjusted cost base of the Shares is the
            Revised Adjusted Cost Base; and

      (c)   the Taxing Authority will accept as effective, an amended Election
            of the parties;

then:

      (d)   the Adjusted Cost Base shall be deemed to have always been, and the
            Adjusted Cost Base wherever referred to in this Agreement, shall be
            read as meaning the Revised Adjusted Cost Base;

      (e)   the Elected Amount shall be deemed to have always been, and the
            Elected Amount wherever referred to in this Agreement shall be read
            as meaning, an amount equal to the Adjusted Elected Amount; and

      (f)   the parties shall take all such steps and jointly execute all such
            documents and elections as may be required and allowed so that the
            aggregate of the amounts elected for purposes of subsection 85(1) of
            the Act with respect to the purchase and sale of the Shares shall be
            equal to the Adjusted Elected Amount.

               ARTICLE 7: VENDOR'S REPRESENTATIONS AND WARRANTIES

7.1   The Vendor represents and warrants to the Purchaser that:

      (a)   on the Closing Date the Shares shall be beneficially owned by the
            Vendor free and clear of all restrictions, options, liens, charges
            and encumbrances, whatsoever, excepting those restrictions set forth
            in the articles of continuance (as amended) of the Corporation, the
            restrictions set forth in the Shareholders Agreement respecting the
            issued and outstanding shares in the capital of the Corporation and
            the restrictions set forth in the Amended and Restated Option
            Agreement dated August 1, 1999, to which the Corporation and the
            Vendor are parties;

      (b)   no person, firm or corporation has any written or verbal agreement,
            option, understanding or commitment or any right or privilege
            capable of becoming an agreement for the purchase of the Shares from
            the Vendor other than as set out in this Agreement, in the articles
            of continuance (as amended) of the Corporation, in the Shareholders
            Agreement respecting the issued and outstanding shares in the
            capital of the Corporation and in


                                      -4-
<PAGE>

            the Amended and Restated Option Agreement dated August 1, 1999 to
            which the Corporation and the Vendor are parties;

      (c)   on the Closing Date the Vendor shall not be a non-resident of Canada
            within the meaning of the Act; and

      (d)   all requisite approvals of the Corporation and its directors and
            shareholders, as applicable, have been obtained so as to permit the
            transfer of the shares from the Vendor to the Purchaser in
            accordance with the terms hereof.

7.2   The representations and warranties contained in section 7.1 shall survive
the completion of the purchase and sale of the Shares and shall continue in full
force and effect for the benefit of the Purchaser; provided always that no claim
with respect to the representations and warranties shall be made by the
Purchaser unless written notice shall have been given to the Vendor within one
(1) year from the Closing Date.

             ARTICLE 8: PURCHASER'S REPRESENTATIONS AND WARRANTIES

8.1   The Purchaser represents and warrants to the Vendor that:

      (a)   the Purchaser is a corporation duly incorporated, validly existing
            and qualified to carry on business under the laws of the Province of
            Alberta;

      (b)   the authorized capital of the Purchaser consists of an unlimited
            number of:

            (i)   Class "A" Common Voting Non-Participating Shares

            (ii)  Class "B" Common Non-Voting Participating Shares

            (iii) Class "C" Common Non-Voting Participating Shares,

            having attached thereto only those rights, privileges, restrictions
            and conditions set forth in the Articles;

      (c)   the issuance to the Vendor of the Share Consideration is valid and
            in accordance with the laws of the Province of Alberta; and

      (d)   this Agreement constitutes a valid and binding obligation of the
            Purchaser and the transactions contemplated by this Agreement are
            not in violation of any terms and conditions of the Articles or any
            agreement to which the Purchaser is a party or by which the
            Purchaser is bound.

8.2   The representations and warranties contained in section 8.1 shall survive
the completion of the purchase and sale of the Shares and shall continue in full
force and effect for the benefit of the Vendor; provided always that no claim
with respect to the representations and


                                      -5-
<PAGE>

warranties shall be made by the Vendor unless written notice shall have been
given to the Purchaser within one (1) year from the Closing Date.

            ARTICLE 9: CLOSING, BENEFICIAL OWNERSHIP AND POSSESSION

9.1   Upon or prior to the Closing Date, or so soon thereafter as the parties
agree upon:

      (a)   the Vendor shall deliver to the Purchaser, share certificates
            representing the Shares, duly endorsed in blank for transfer and
            shall cause transfers of such Shares to be duly recorded in the
            books of the Corporation in the name of the Purchaser or its
            nominee;

      (b)   the Purchaser shall:

            (i)   issue and deliver to the Vendor the Share Consideration; and

            (ii)  at the request of the Vendor, deliver to the Vendor a copy of
                  the resolution of the board of directors of the Purchaser
                  adding the Stated Capital Addition to the stated capital
                  account maintained for the shares forming the Share
                  Consideration, certified by the secretary of the Purchaser as
                  a true and exact copy, where required by the Vendor.

9.2   If the corporate proceedings required to duly and validly record the
Shares in the name of the Purchaser or its nominee have not been completed on
the Closing Date, the Vendor agrees that, until such time as the Shares shall be
recorded in the name of the Purchaser or its nominee, the Vendor shall stand as
owner of the Shares as bare trustee for and on behalf of the Purchaser as the
beneficial owner, and in such capacity the Vendor shall:

      (a)   receive on behalf of, account to and forthwith pay over to the
            Purchaser, any and all dividends and other amounts received by the
            Vendor as the recorded owner of the Shares; and

      (b)   transfer and deal with the Shares in such manner as the Purchaser
            may direct.

The Purchaser shall at all times hereafter indemnify and keep indemnified the
Vendor against all liabilities which the Vendor may incur by reason of the
Shares being held in trust in the name of the Vendor for the benefit of the
Purchaser.

                    ARTICLE 10: FURTHER ACTS AND ASSURANCES

10.1  Each of the parties shall, upon the reasonable request of the other party,
make, do, execute or cause to be made, done, or executed all such further and
other lawful acts, deeds,


                                      -6-
<PAGE>

things, documents and assurances of whatsoever nature and kind for the better
performance of the terms and conditions of this Agreement.

                              ARTICLE 11: NOTICES

11.1  Any notice, direction or other instrument required or permitted to be
given under the provisions of this Agreement shall be in writing and either
delivered personally, sent by prepaid registered mail or sent by facsimile to
the party to be notified. The addresses of the parties for such purpose shall
respectively be:

      (a)   to the Vendor at its registered office address; and

      (b)   to the Purchaser at its registered office address;

or to such other address in Canada of which either party may from time to time
notify the other.

11.2  Any notice, direction or other instrument shall:

      (a)   if delivered, be deemed to have been given or received on the day on
            which it was so delivered and if not on a business day then on the
            business day next following the day of delivery;

      (b)   if sent by facsimile shall be deemed to have been given or received
            on the same business day as it was sent, provided facsimile
            confirmation of receipt is received by the sender;

      (c)   if mailed, shall be deemed to have been given or received on the
            third day following the day on which it was mailed; and

      (d)   if mailed at a time when there is an interruption or an anticipated
            interruption of mail service affecting the delivery of such mail,
            shall be deemed to have been given or received on the fifth business
            day after the date that normal postal service is restored.

                          ARTICLE 12: ENTIRE AGREEMENT

12.1  This Agreement constitutes and contains the entire agreement between the
parties respecting the subject matter of this Agreement and supersedes any prior
agreements whether written or verbal.


                                      -7-
<PAGE>

                             ARTICLE 13: ENUREMENT

13.1  This Agreement shall enure to the benefit of and be binding upon the
parties and their respective successors, personal representatives and assigns.

                            ARTICLE 14: COUNTERPART

14.1  This Agreement may be executed in counterparts with the same effect as if
all of the parties hereto had signed the same document. All counterparts shall
be construed together and shall constitute one and the same agreement. A
facsimile transmitted copy of this Agreement signed by a party, in counterpart
or otherwise, shall be deemed to be evidence of a properly executed, delivered
and binding agreement of the party so signing, notwithstanding any variation in
the actual dates of execution. The parties each agree to promptly return an
original duly executed counterpart of this Agreement following the transmission
of the facsimile transcribed copy thereof.



      IN WITNESS WHEREOF the parties have properly executed this Agreement, as
of the Effective Date.

                                            WORLDWIDE FIBER HOLDINGS LTD.

                                            PER:
                                                --------------------------------



                                            URBANLINK HOLDINGS LTD.

                                            PER:
                                                --------------------------------


                                      -8-
<PAGE>

                                  SCHEDULE "A"
                                  ------------

           TO THAT CERTAIN ROLL-OVER AGREEMENT MADE BETWEEN WORLDWIDE
          FIBER HOLDINGS LTD. (AS VENDOR) AND URBANLINK HOLDINGS LTD.
                                 (AS PURCHASER)
          -----------------------------------------------------------


                                     SHARES
                                     ------


Share Certificate # _____ representing ___________________________ Subordinate
Voting Shares of 360networks inc.


<PAGE>

                                  SCHEDULE "B"
                                  ------------

           TO THAT CERTAIN ROLL-OVER AGREEMENT MADE BETWEEN WORLDWIDE
          FIBER HOLDINGS LTD. (AS VENDOR) AND URBANLINK HOLDINGS LTD.
                                 (AS PURCHASER)
          -----------------------------------------------------------

                              SHARE CONSIDERATION
                              -------------------


Share Certificate # 1C representing 490,000 Class "C" Common Non-Voting
Participating Shares in the capital of URBANLINK HOLDINGS Ltd.