Joinder Agreement to Securities Exchange Agreement among iBasis, Inc., iBasis Global, Inc., U.S. Bank National Association, and Acceding Holders (May 29, 2003)

Exhibit 10.57

JOINDER AGREEMENT

        Reference is hereby made to the Securities Exchange Agreement dated as of February 21, 2003 (the "Exchange Agreement"), by and between the iBasis, Inc. (the "Company"), iBasis Global, Inc. ("Global" and, together with the Company, the "Borrower"), iBasis Securities Corporation, JMG Triton Offshore Fund Limited CITCO and the other Exchanging Holders named therein, and U.S. Bank National Association, as Collateral Agent, Fiscal Agent and Warrant Agent (the "Agent"). Capitalized terms used herein and not otherwise defined shall have the meanings assigned to such terms in the Exchange Agreement.

        Pursuant to the terms and conditions of the Exchange Agreement, the Borrower, the Agent and the persons set forth on Annex A hereto (the "Acceding Holders" and each, individually, an "Acceding Holder") and each person that becomes an Acceding Holder to this Joinder Agreement pursuant to Section 9 hereof hereby agree as follows:

        SECTION 1.    Subject to the terms and conditions of this Joinder Agreement, dated as of May 29, 2003 (this "Joinder Agreement"), each Acceding Holder hereby agrees to exchange, on or as of the Effective Date (as defined below), Convertible Notes in the aggregate principal amount set forth opposite such Acceding Holder's name on Annex A hereto for (a) the aggregate principal amount set forth opposite such Acceding Holder's name on Annex A hereto of the Borrower's 11.5% Senior Secured Notes due 2005 and (b) Warrants exercisable for an aggregate number of shares of Common Stock set forth opposite such Acceding Holder's name on Annex A hereto, all in accordance with the terms and conditions set forth in the Exchange Agreement. Each Acceding Holder hereby agrees to be bound by, and hereby requests the agreement of the Borrower and the Agent that such Acceding Holder shall be entitled to the benefits of, all of the terms, conditions and provisions of the Exchange Agreement as if such Acceding Holder had been one of the Persons originally executing the Exchange Agreement as an "Exchanging Holder"; provided that nothing herein shall be construed as making such Acceding Holder liable to the Borrower or the other Exchanging Holders or the Borrower liable to such Acceding Holder in respect of any acts or omissions of any party to the Exchange Agreement or in respect of any other event occurring prior to the Effective Date (as defined below) of this Joinder Agreement. Furthermore, each Acceding Holder expressly acknowledges and agrees that by executing and delivering this Joinder Agreement (i) it becomes a party to the Exchange Agreement, the Intercreditor Agreement and the Subordination Agreement, in each case as an "Exchanging Holder", (ii) it accepts and shall be subject to the terms of the Fiscal Agency Agreement and the Warrant Agreement as a Noteholder and a Warrantholder, respectively, and (iii) it accepts and shall be subject to the terms of each of the Credit Documents as an Exchanging Holder, as the same may be applicable.

        SECTION 2.    (a) Each Acceding Holder, subject to Section 2(b) below, severally, but not jointly with any other Acceding Holder, (i) represents and warrants that (A) it is duly and legally authorized to enter into this Joinder Agreement, (B) the execution, delivery and performance of this Joinder Agreement do not conflict with any provision of law or, if such Acceding Holder is not an individual, of the charter or by-laws of such Acceding Holder, or of any agreement binding on such Acceding Holder, and (C) all acts, conditions and things required to be done and performed and to have occurred prior to the execution, delivery and performance of this Joinder Agreement, and to render the same the legal, valid and binding obligation of such Acceding Holder, enforceable against it in accordance with its terms, have been done and performed and have occurred in due and strict compliance with all applicable laws; (ii) confirms that it has received a copy of the Exchange Agreement, together with such other documents and information as it has deemed appropriate to make its own credit analysis and decision to enter into this Joinder Agreement; (iii) agrees that it will, independently and without reliance upon the Exchanging Holders or the Agent and based on such documents and information as it shall deem appropriate at the time, continue to make its own credit decisions in taking or not taking action under the Exchange Agreement; (iv) appoints and authorizes the Agent to take such action as

agent on its behalf and to exercise such powers under the Exchange Agreement and the other Loan Documents as are delegated to the Agent by the terms thereof, together with such powers as are reasonably incidental thereto; and (v) agrees that it will perform in accordance with their terms all of the obligations which by the terms of the Exchange Agreement are required to be performed by it as an Exchanging Holder.

        (b) Each Acceding Holder, severally, but not jointly with any other Acceding Holder, represents and warrants only as to itself that, except as set forth on Schedule 2 attached hereto, it is an "accredited investor" as that term is defined in Rule 501 of Regulation D promulgated under the Securities Act, by virtue, inter alia, of its being (i) a corporation, Massachusetts or similar business trust, or partnership, not formed for the specific purpose of acquiring the Warrants or the shares of Common Stock issuable upon exercise of the Warrants, with total assets in excess of $5,000,000, (ii) a natural person whose individual net worth, or joint net worth with that person's spouse, at the time of his or her exchange exceeds $1,000,000, or (iii) a natural person who had an individual income in excess of $200,000 in each of the two most recent years or joint income with that person's spouse in excess of $300,000 in each of those years and has a reasonable expectation of reaching the same income level in the current year. The Company hereby waives the accredited investor representation set forth in Section 8.5 of the Exchange Agreement for those Acceding Holders set forth on Schedule 2.

        SECTION 3.    The Acceding Holders hereby request that the Borrower issue a new global note increasing the principal amount by $6,100,000 and a new global warrant increasing the number of shares exercisable thereunder by an aggregate of 1,116,605 shares of Common Stock; and the Company hereby authorizes and directs the Agent to authenticate and register, in the names and denominations specified herein, and to deliver each such Global Security, when so authenticated and registered, to The Depository Trust Company, or its custodian, on the Effective Date (as defined below).

        SECTION 4.    The effective date for this Joinder Agreement shall be May 29, 2003 (the "Effective Date"). Following the execution of this Joinder Agreement by the Borrower and the Acceding Holders, it will be delivered to the Agent for acceptance.

        SECTION 5.    By execution of this Joinder Agreement, the Borrower hereby certifies that: (i) the sum of the aggregate original principal amount of the Notes to be issued pursuant to this Joinder Agreement, together with the aggregate original principal amount of all other Notes previously issued under the Fiscal Agency Agreement, does not exceed $28,750,000; (ii) the sum of the aggregate amount of Warrant Shares issuable upon exercise of the Warrants to be issued pursuant to this Joinder Agreement, together with the aggregate amount of Warrant Shares issuable (or, if exercised, issued) upon exercise of all Warrants previously issued pursuant to any and all other Joinder Agreements entered into pursuant to (and after the date of) the Exchange Agreement (which, for avoidance of doubt, does not include any Warrants issued to the Exchanger, as that term is defined in and pursuant to the Exchange Agreement, or any Warrants issued pursuant to the Symphony Exchange Agreement) does not exceed 1,967,798; and (iii) this Joinder Agreement, and the Notes and Warrants to be issued pursuant thereto, constitutes a Permitted Exchange in accordance with Section 9.3 of, and otherwise is in accordance with, the Exchange Agreement.

        SECTION 6.    THIS JOINDER AGREEMENT SHALL FOR ALL PURPOSES BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS (WITHOUT REGARD TO THE PRINCIPLES OF CONFLICTS OF LAWS).

        SECTION 7.    The use in this Joinder Agreement of neuter pronouns in reference to a party hereto shall be deemed to include the masculine or feminine, as the context may require.

        SECTION 8.    This Joinder Agreement may be executed in any number of counterparts which shall together constitute but one and the same agreement. This Joinder Agreement, once duly executed

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and delivered in accordance with the terms of the Exchange Agreement, may be relied upon conclusively by the Agent.

        SECTION 9.    Any person that is a holder of Convertible Notes may become an Acceding Holder party to this Joinder Agreement by executing and delivering to the Borrower and the Agent, on or prior to June 30, 2003, an Instrument of Accession, substantially in the form of Annex B attached hereto, provided that the Borrower and the Agent consent to such person becoming an Acceding Holder party to this Joinder Agreement (which consent shall be conclusively deemed to have been given by the Borrower and Agent when each countersigns the Instrument of Accession executed by such person). Any such Instrument of Accession executed by any such person and countersigned by the Borrower and Agent shall become a part of this Joinder Agreement. It is hereby understood and agreed that each of Annex A and Schedule 2 shall be updated to reflect such Acceding Holder's interest. Any exchange of Convertible Notes for Notes by a holder of Convertible Notes who becomes an Acceding Party to this Joinder Agreement shall be deemed to have occurred for all purposes of this Joinder Agreement and such Convertible Notes as of the Effective Date, regardless of the date on which exchange may actually be consummated.

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        IN WITNESS WHEREOF, intending to be legally bound, each of the undersigned has caused this Joinder Agreement to be executed on its behalf by its officer thereunto duly authorized, to take effect as a sealed instrument as of the date first above written.

Annex A

Schedule 2

Schedule of Exceptions

Annex B

Instrument of Accession

[June    , 2003]

        Reference is made to that certain Joinder Agreement dated as of May 29, 2003, a copy of which is attached hereto (as amended and in effect from time to time, the "Joinder Agreement"), among iBasis, Inc. (the "Company"), iBasis Global, Inc. ("Global" and together with the Company, the "Borrower"), the Acceding Holders as named therein, and U.S. Bank National Association, as Collateral Agent, Fiscal Agent and Warrant Agent (the "Agent").

        The persons listed on Schedule 1 attached hereto are the owners or holders of the Company's 5³/4% Convertible Subordinated Notes due 2005 and each person hereby agrees that, by his or her execution hereof, that it is an Acceding Holder party to the Joinder Agreement and that he or she is subject to all of the restrictions and conditions applicable to the Acceding Holders set forth in such Joinder Agreement. This Instrument of Accession shall take effect and shall become a part of said Joinder Agreement immediately upon acceptance thereof by the Borrower and the Agent. Attached hereto are revised Annex A and Schedule 2 to the Joinder Agreement.

        Executed as of the date set forth below under the laws of the Commonwealth of Massachusetts.

Schedule 1

[List the Acceding Holders that are additional parties to the Joinder Agreement pursuant to Section 9 thereof.]

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Exhibit 10.57

Annex A

Schedule 2

Annex B

Schedule 1