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 onAnnex Ahereto for (a) the aggregate principal amount set
forth opposite such Acceding Holder's name onAnnex Ahereto 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 onAnnex Ahereto, 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";providedthat 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 2attached 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

2

---

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 ofAnnex
Battached 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 ofAnnex Aand Schedule 2shall 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.

[Remainder
of Page Intentionally Left Blank]

3

---

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.

| |  | iBASIS, INC.       
| |  | -------------------
| |  | By:                
| |  | iBASIS GLOBAL, INC.
| |  | By:                

| |  | ACCEDING HOLDERS
listed onAnnex Ahereto
| |  | ---------------------------------------
| |  | By:                                    
| |  |                                        

|Acknowledged this 29 day of May, 2003:                                                      |                                                           
|------------------------------------------------------------------------------------------- | ----------------------------------------------------------
|U.S. BANK NATIONAL ASSOCIATION,as Collateral Agent under the Securities Exchange Agreement. |                                                           
|By:                                                                                         | /s/JOHN BRENNANName:  John Brennan
Title:    Trust Officer

---

Annex A

|Name of Acceding Holder         |  | Principal Amount of exchanged Convertible Notes |         | Principal Amount of new Notes |  | Number of Shares into which Warrants are exercisable
|------------------------------- |  | ----------------------------------------------- | ------- | ----------------------------- |  | ----------------------------------------------------
|Ferguson, William & Janiece B.  |  |                                                 | 70,000  |                               |  | 35000                                               
|Anepohl, Clarilynn D.           |  |                                                 | 45,000  |                               |  | 22000                                               
|Breaux, Frank W.                |  |                                                 | 100,000 |                               |  | 50000                                               
|Anepohl, Jr., Fred G.           |  |                                                 | 35,000  |                               |  | 18000                                               
|Solar, Curtis J & Kay M         |  |                                                 | 20,000  |                               |  | 10000                                               
|Attrep, John                    |  |                                                 | 90,000  |                               |  | 45000                                               
|Balcom, Grace, Randy & Carolyn  |  |                                                 | 75,000  |                               |  | 37000                                               
|Ballard, E. Dwight              |  |                                                 | 70,000  |                               |  | 35000                                               
|Ballard, Dwight & Sandra Jean   |  |                                                 | 20,000  |                               |  | 10000                                               
|Balle, David S                  |  |                                                 | 35,000  |                               |  | 18000                                               
|Balliro, Peggy G                |  |                                                 | 20,000  |                               |  | 10000                                               
|Williams, John Kelly            |  |                                                 | 50,000  |                               |  | 25000                                               
|Bayard, William J               |  |                                                 | 25,000  |                               |  | 13000                                               
|Anjier, Joseph L                |  |                                                 | 90,000  |                               |  | 45000                                               
|Bombet MD, Leon H               |  |                                                 | 100,000 |                               |  | 50000                                               
|Borak, Rachel                   |  |                                                 | 35,000  |                               |  | 18000                                               
|Brewer, Ralph                   |  |                                                 | 65,000  |                               |  | 32000                                               
|Bowman, Edwin A                 |  |                                                 | 35,000  |                               |  | 18000                                               
|Burroughs MD, Richard           |  |                                                 | 175,000 |                               |  | 87000                                               
|Hirsch, Gary S. & Karen E       |  |                                                 | 75,000  |                               |  | 37000                                               
|Hirsch, Gary S                  |  |                                                 | 130,000 |                               |  | 65000                                               
|Weldon, William E               |  |                                                 | 170,000 |                               |  | 85000                                               
|Weldon, William & Cornelia      |  |                                                 | 115,000 |                               |  | 57000                                               
|Newcomer, JR, George C          |  |                                                 | 90,000  |                               |  | 45000                                               
|Cassidy, Laura                  |  |                                                 | 40,000  |                               |  | 20000                                               
|Pace, William Robert            |  |                                                 | 70,000  |                               |  | 35000                                               
|CJS Investments                 |  |                                                 | 25,000  |                               |  | 13000                                               
|Claiborne, Earl B.              |  |                                                 | 110,000 |                               |  | 55000                                               
|Claiborne, Earl B. & Ethel L.   |  |                                                 | 25,000  |                               |  | 13000                                               
|Head & Neck P/S Plan, Clinic of |  |                                                 | 30,000  |                               |  | 15000                                               
|Head & Neck Pension Pln, Clinic |  |                                                 | 65,000  |                               |  | 32000                                               
|Collins, Steven & Eileen        |  |                                                 | 30,000  |                               |  | 15000                                               
|De Fraites, Rebecca Kay         |  |                                                 | 20,000  |                               |  | 10000                                               
|de Jongh, Maria P               |  |                                                 | 20,000  |                               |  | 10000                                               
|De Fraites JR, Emanuel G        |  |                                                 | 50,000  |                               |  | 25000                                               
|de Jongh, Alberto & Maria       |  |                                                 | 160,000 |                               |  | 80000                                               
|Delhaye Children's Tr II        |  |                                                 | 65,000  |                               |  | 33000                                               
|Mayfield, Mary Jo               |  |                                                 | 115,000 |                               |  | 57000                                               
|Dement, Leon D. & Sylvia L.     |  |                                                 | 75,000  |                               |  | 37000                                               
|Delhaye, Mary Field             |  |                                                 | 25,000  |                               |  | 13000                                               
|Dupont, J. Benton               |  |                                                 | 115,000 |                               |  | 57000                                               
|Dupont MD, J. Benton            |  |                                                 | 30,000  |                               |  | 15000                                               
|Dyer, Herbert R.                |  |                                                 | 180,000 |                               |  | 90000                                               
|Phillips, Gary N                |  |                                                 | 210,000 |                               |  | 105000                                              
|Silverberg, Joel D              |  |                                                 | 25,000  |                               |  | 13000                                               
|Trahan III, Victor "Trey"       |  |                                                 | 130,000 |                               |  | 65000                                               
|Barry, Mary Kathleen V          |  |                                                 | 200,000 |                               |  | 100000                                              
|Cloud, Louise C                 |  |                                                 | 60,000  |                               |  | 30000                                               

---

|Fazio, Frank L                   |  |  | 45,000  |  |  | 23000  |  | 4119 
|-------------------------------- |  |  | ------- |  |  | ------ |  | -----
|Ferguson, William F              |  |  | 50,000  |  |  | 25000  |  | 4576 
|Mayfield FP, Field               |  |  | 65,000  |  |  | 32000  |  | 5949 
|Sanders, Donald Leroy            |  |  | 75,000  |  |  | 37000  |  | 6864 
|Fleury, JR, Linton Albert        |  |  | 55,000  |  |  | 27000  |  | 5034 
|Gasparrini, Sue D                |  |  | 25,000  |  |  | 13000  |  | 2288 
|Gasparrini, William G            |  |  | 65,000  |  |  | 32000  |  | 5949 
|Mayfield, Drew Generation Skippi |  |  | 10,000  |  |  | 5000   |  | 915  
|Greco, Leu Anne                  |  |  | 50,000  |  |  | 25000  |  | 4576 
|Greco TTEE, Cyrus John           |  |  | 100,000 |  |  | 50000  |  | 9153 
|Grissom MD, Robert T             |  |  | 25,000  |  |  | 13000  |  | 2288 
|Grissom MD, Robert T             |  |  | 15,000  |  |  | 8000   |  | 1373 
|Greco, Cyrus John                |  |  | 120,000 |  |  | 60000  |  | 10983
|Haga, D Bruce                    |  |  | 25,000  |  |  | 13000  |  | 2288 
|Hall MD, Donald Leroy            |  |  | 110,000 |  |  | 55000  |  | 10068
|Kennon JR, Robert F              |  |  | 70,000  |  |  | 35000  |  | 6407 
|Harner, James Oliver             |  |  | 70,000  |  |  | 35000  |  | 6407 
|Hirsch, Karen Elkind             |  |  | 30,000  |  |  | 15000  |  | 2746 
|Haydel, Froisin J.               |  |  | 220,000 |  |  | 110000 |  | 20127
|Haydel, F. Joseph & Frances L.   |  |  | 50,000  |  |  | 25000  |  | 4576 
|Mayfield Trust, John D.          |  |  | 25,000  |  |  | 13000  |  | 2288 
|Bayard, William J & Peggy G      |  |  | 30,000  |  |  | 15000  |  | 2746 
|Bayard, William J & Peggy G      |  |  | 15,000  |  |  | 8000   |  | 1373 
|Norman, Joseph R.                |  |  | 95,000  |  |  | 47000  |  | 8695 
|Norman, Joseph R. & Susan        |  |  | 15,000  |  |  | 8000   |  | 1373 
|BCL Investment Partnership       |  |  | 90,000  |  |  | 45000  |  | 8237 
|Kirsch JR, William G             |  |  | 40,000  |  |  | 20000  |  | 3661 
|Herad, Khosrow                   |  |  | 55,000  |  |  | 27000  |  | 5034 
|Trahan III "Trey", Victor F      |  |  | 10,000  |  |  | 5000   |  | 915  
|Laville, Anne                    |  |  | 50,000  |  |  | 25000  |  | 4576 
|Laville, Anne M.                 |  |  | 40,000  |  |  | 20000  |  | 3661 
|Lee, David M                     |  |  | 40,000  |  |  | 20000  |  | 3661 
|Lee, J. David                    |  |  | 100,000 |  |  | 50000  |  | 9153 
|Lockwood, Thomas M.              |  |  | 25,000  |  |  | 13000  |  | 2288 
|La. Realtors Assoc LDF           |  |  | 50,000  |  |  | 25000  |  | 4576 
|Delhaye Trust, Mary Jo           |  |  | 10,000  |  |  | 5000   |  | 915  
|Maestri Jr., Natale A            |  |  | 25,000  |  |  | 13000  |  | 2288 
|Mayfield, Lucy R                 |  |  | 40,000  |  |  | 20000  |  | 3661 
|Mayfield, Mary Jane              |  |  | 95,000  |  |  | 47000  |  | 8695 
|Cloyd, William P.                |  |  | 110,000 |  |  | 55000  |  | 10068
|McMillin, Bill                   |  |  | 70,000  |  |  | 35000  |  | 6407 
|McNeil, Edwin B                  |  |  | 50,000  |  |  | 25000  |  | 4576 
|Mosley, Guy J & Wanda M          |  |  | 60,000  |  |  | 30000  |  | 5492 
|Mosley, Guy J                    |  |  | 30,000  |  |  | 15000  |  | 2746 
|Mosley, Wanda M                  |  |  | 40,000  |  |  | 20000  |  | 3661 
|Myres, Betty G                   |  |  | 50,000  |  |  | 25000  |  | 4576 
|Myres, Yancey C                  |  |  | 45,000  |  |  | 22000  |  | 4119 
|Shannon, Keith D                 |  |  | 40,000  |  |  | 20000  |  | 3661 
|Noland, Ameila M                 |  |  | 50,000  |  |  | 25000  |  | 4576 
|Ordeneaux, Jerry C.              |  |  | 140,000 |  |  | 70000  |  | 12814

---

|Poche, Francis Jay              |  |  | 15,000  |  |  | 8000   |  | 1373 
|------------------------------- |  |  | ------- |  |  | ------ |  | -----
|Pollard, Glenda S               |  |  | 65,000  |  |  | 32000  |  | 5949 
|Reine MD, Georgia A             |  |  | 20,000  |  |  | 10000  |  | 1831 
|Reine, Georgia A                |  |  | 175,000 |  |  | 87000  |  | 16017
|Richey, Anne Q                  |  |  | 70,000  |  |  | 35000  |  | 6407 
|Richey, Charles A               |  |  | 70,000  |  |  | 35000  |  | 6407 
|Richey, Charles A & Anne Q      |  |  | 60,000  |  |  | 30000  |  | 5492 
|Coleman & Partners, Robert      |  |  | 100,000 |  |  | 50000  |  | 9153 
|Russo III DDS, Camille Joseph   |  |  | 60,000  |  |  | 30000  |  | 5492 
|Russo, Camille J & Patricia     |  |  | 50,000  |  |  | 25000  |  | 4576 
|Russo III DDS MPP, Camille J    |  |  | 90,000  |  |  | 45000  |  | 8237 
|Rucci, Peter J                  |  |  | 40,000  |  |  | 20000  |  | 3661 
|Schexnayder, Michael C. & Charl |  |  | 25,000  |  |  | 13000  |  | 2288 
|Schexnayder MD, Michael C.      |  |  | 60,000  |  |  | 30000  |  | 5492 
|Schucker, Robert Charles        |  |  | 170,000 |  |  | 85000  |  | 15559
|Schwartzberg, Harvey J          |  |  | 70,000  |  |  | 35000  |  | 6407 
|Schwartzberg, Harvey J & Betty  |  |  | 25,000  |  |  | 13000  |  | 2288 
|Schwartzberg, Ellis J. & Barb   |  |  | 55,000  |  |  | 27000  |  | 5034 
|Shannon, Maureen G              |  |  | 50,000  |  |  | 25000  |  | 4576 
|Drake, Kathryn M                |  |  | 15,000  |  |  | 8000   |  | 1373 
|Cloyd MD, William P.            |  |  | 210,000 |  |  | 105000 |  | 19220
|Solar, Karen Kay                |  |  | 30,000  |  |  | 15000  |  | 2746 
|Mayfield Trust, Lucy R.         |  |  | 25,000  |  |  | 12000  |  | 2288 
|Steen, Gayle                    |  |  | 175,000 |  |  | 87000  |  | 16017
|Collins TTEE, Steven D          |  |  | 50,000  |  |  | 25000  |  | 4576 
|Stark, Michael M                |  |  | 80,000  |  |  | 40000  |  | 7322 
|Wood, Danny S.                  |  |  | 15,000  |  |  | 8000   |  | 1373 
|Cavalier, MD, Steven J.         |  |  | 110,000 |  |  | 55000  |  | 10068
|Alvarez, MD, Frank J.           |  |  | 100,000 |  |  | 50000  |  | 9153 
|Schwartzberg, MD, Glen          |  |  | 85,000  |  |  | 42000  |  | 7780 
|Robin, MD, Richard L.           |  |  | 140,000 |  |  | 70000  |  | 12814
|Roberts, MD, Floyd J.           |  |  | 165,000 |  |  | 82000  |  | 15102
|Melker MD, Merritt "Trey"       |  |  | 100,000 |  |  | 50000  |  | 9153 
|Dement, MD, Leon N.             |  |  | 190,000 |  |  | 95000  |  | 17390
|Gladney, MD, William W.         |  |  | 100,000 |  |  | 50000  |  | 9153 
|Peairs, MD, Patricia T.         |  |  | 35,000  |  |  | 18000  |  | 3203 
|Hargus, MD, Louis Field         |  |  | 125,000 |  |  | 62000  |  | 11441
|Waggenspack, MD, Mark           |  |  | 70,000  |  |  | 35000  |  | 6407 
|Silverberg, MD, Joel            |  |  | 25,000  |  |  | 13000  |  | 2288 
|Rolfsen, MD, Michael            |  |  | 80,000  |  |  | 40000  |  | 7322 
|Seiter, MD, Elizabeth D.        |  |  | 90,000  |  |  | 45000  |  | 8237 
|Vicari, MD, Roberta C.          |  |  | 50,000  |  |  | 25000  |  | 4576 
|Vath, MD, Richard R.            |  |  | 105,000 |  |  | 52000  |  | 9610 
|Silvey, Edgar H.                |  |  | 80,000  |  |  | 40000  |  | 7322 
|Hecker MD, Roger                |  |  | 35,000  |  |  | 17000  |  | 3203 
|Earhart MD, Robert N            |  |  | 115,000 |  |  | 57000  |  | 10525
|Vinci, MD, Edmund               |  |  | 60,000  |  |  | 30000  |  | 5492 
|Fraser, MD, Warren M.           |  |  | 125,000 |  |  | 62000  |  | 11441
|Thudium, Jr., John F.           |  |  | 165,000 |  |  | 82000  |  | 15102
|Barry, Mary Kathleen            |  |  | 35,000  |  |  | 18000  |  | 3203 

---

|Garcia, O J                                                                                                                                                                |  |   | 65,000                    |  |   | 32000     |  | 5949     
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |  | - | ------------------------- |  | - | --------- |  | ---------
|Waguespack, Hubert J.                                                                                                                                                      |  |   | 140,000                   |  |   | 70000     |  | 12814    
|Waguespack, Phillip J.                                                                                                                                                     |  |   | 100,000                   |  |   | 50000     |  | 9153     
|Wall IV, Williams D                                                                                                                                                        |  |   | 45,000                    |  |   | 23000     |  | 4119     
|Wall IV, Williams D                                                                                                                                                        |  |   | 30,000                    |  |   | 15000     |  | 2746     
|Walsh, Robert Eugene                                                                                                                                                       |  |   | 20,000                    |  |   | 10000     |  | 1831     
|Walsh, Robert & Linda                                                                                                                                                      |  |   | 50,000                    |  |   | 25000     |  | 4576     
|Williams, John Kelly                                                                                                                                                       |  |   | 80,000                    |  |   | 40000     |  | 7322     
|Wood, Danny S. & Leah S.                                                                                                                                                   |  |   | 25,000                    |  |   | 13000     |  | 2288     
|Wood, MD, Danny S.                                                                                                                                                         |  |   | 110,000                   |  |   | 55000     |  | 10068    
|Bernhard, Jo'An O                                                                                                                                                          |  |   | 40,000                    |  |   | 20000     |  | 3661     
|Zuckerman, Steven J.                                                                                                                                                       |  |   | 50,000                    |  |   | 25000     |  | 4576     
|Zuckerman, Steven & Marialden                                                                                                                                              |  |   | 45,000                    |  |   | 23000     |  | 4119     
|Allen, Lane                                                                                                                                                                |  |   | 65,000                    |  |   | 33000     |  | 5949     
|Ashford, Jules                                                                                                                                                             |  |   | 110,000                   |  |   | 55000     |  | 10068    
|Bradford, Juanita                                                                                                                                                          |  |   | 75,000                    |  |   | 37000     |  | 6864     
|Ellison, Linda                                                                                                                                                             |  |   | 50,000                    |  |   | 25000     |  | 4576     
|Gust, Dorothy                                                                                                                                                              |  |   | 10,000                    |  |   | 5000      |  | 915      
|Gust, Richard                                                                                                                                                              |  |   | 65,000                    |  |   | 32000     |  | 5949     
|Salter, Barbara                                                                                                                                                            |  |   | 45,000                    |  |   | 23000     |  | 4119     
|Summers, Steven                                                                                                                                                            |  |   | 30,000                    |  |   | 15000     |  | 2746     
|Summers, Jane                                                                                                                                                              |  |   | 10,000                    |  |   | 5000      |  | 915      
|Summers, S & J                                                                                                                                                             |  |   | 35,000                    |  |   | 17000     |  | 3203     
|TOTAL:                                                                                                                                                                     |  | $ | 11,770,000                |  | $ | 5,884,000 |  | 1,077,248
|Additional New Notes and Warrants to be held by iBasis until such time as additional Acceding Holders become party to this Joinder Agreement pursuant to Section 9 hereof. |  | $ | 430,000
To be exchanged. |  | $ | 216,000   |  | 39,357   
|TOTAL:                                                                                                                                                                     |  |   | 12,200,000                |  | $ | 6,100,000 |  | 1,116,605

---

Schedule 2

Schedule of Exceptions

|Name of
Acceding Holder  |  | Principal Amount
of exchanged
Convertible Notes |  | Principal Amount
of new Notes |  | Number of Shares
into which Warrants
are exercisable
|------------------------ |  | ----------------------------------------------- |  | ----------------------------- |  | ----------------------------------------------------
|Allen, Lane              |  | 65,000                                          |  | 33,000                        |  | 5,949                                               
|Ashford, Jules           |  | 110,000                                         |  | 55,000                        |  | 10,068                                              
|Bradford, Juanita        |  | 75,000                                          |  | 37,000                        |  | 6,864                                               
|Ellison, Linda S         |  | 50,000                                          |  | 25,000                        |  | 4,576                                               
|Gust, Dorothy B          |  | 10,000                                          |  | 5,000                         |  | 915                                                 
|Gust, Richard            |  | 65,000                                          |  | 32,000                        |  | 5,949                                               
|Salter, Barbara G        |  | 45,000                                          |  | 23,000                        |  | 4,119                                               
|Summers, Steven Hall     |  | 30,000                                          |  | 15,000                        |  | 2,746                                               
|Summers, Jane Kay        |  | 10,000                                          |  | 5,000                         |  | 915                                                 
|Summers, Steven & Jane K |  | 35,000                                          |  | 17,000                        |  | 3,203                                               

---

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 onSchedule 1attached 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 revisedAnnex Aand Schedule 2to the Joinder Agreement.

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

| |  | ACCEDING HOLDERS
listed onSchedule 1hereto
| |  | ------------------------------------------
| |  | By:                                       
| |  | By:                                       

|Accepted: iBASIS, INC. |             
|---------------------- | ------------
|By:                    | Name:
Title:

---

|iBASIS GLOBAL, INC.                                                                         |             
|------------------------------------------------------------------------------------------- | ------------
|By:                                                                                         | Name:
Title:
|U.S. BANK NATIONAL ASSOCIATION,as Collateral Agent under the Securities Exchange Agreement. |             
|By:                                                                                         | Name:
Title:

---

Schedule 1

|Name of
Acceding Holder |  | Principal Amount
of exchanged
Convertible Notes |  | Principal Amount
of new Notes |  | Number of Shares
into which Warrants
are exercisable
|----------------------- |  | ----------------------------------------------- |  | ----------------------------- |  | ----------------------------------------------------

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

---

QuickLinks