<PAGE>


                                                                    Exhibit 4.8


===============================================================================



                          iBASIS SECURITIES CORPORATION


                                    GUARANTEE


                          Dated as of January 30, 2003



               U.S. Bank National Association, as Collateral Agent



===============================================================================


<PAGE>


                                TABLE OF CONTENTS

<TABLE>
<S>     <C>                                                                                              <C>
1.       Reference to Exchange Agreement; Definitions; Certain Rules of Construction.......................  1
         1.1.     "Agreement"..............................................................................  1
         1.2.     "Obligors"...............................................................................  1
2.       Guarantee.........................................................................................  1
         2.1.     Guarantee of Obligations.................................................................  1
         2.2.     Continuing Obligation....................................................................  2
         2.3.     Waivers with Respect to Obligations......................................................  2
         2.4.     Investors' Power to Waive, etc...........................................................  4
         2.5.     Information Regarding the Borrower, etc..................................................  4
         2.6.     Certain Guarantor Representations........................................................  5
         2.7.     Subrogation..............................................................................  5
         2.8.     Subordination............................................................................  6
         2.9.     Future  Subsidiaries; Further Assurances.................................................  6
3.       Representations and Warranties....................................................................  6
         3.1.     Organization and Business................................................................  6
         3.2.     Authorization and Enforceability.........................................................  7
         3.3.     No Legal Obstacle to Agreements..........................................................  7
         3.4.     Litigation...............................................................................  7
4.       Successors and Assigns............................................................................  8
5.       Notices...........................................................................................  8
6.       Reimbursement of Expenses.........................................................................  8
7.       Venue; Service of Process.........................................................................  8
8.       WAIVER OF JURY TRIAL..............................................................................  9
9.       General...........................................................................................  9
</TABLE>


                                       i


<PAGE>


                          iBASIS SECURITIES CORPORATION


                                    GUARANTEE

         This Agreement, dated as of January 30, 2003, is among iBasis
Securities Corporation, a Massachusetts corporation (the "GUARANTOR"), and U.S.
Bank National Association, as Collateral Agent (the "COLLATERAL AGENT") for
itself and the Holders under the Exchange Agreement (as defined below). The
parties agree as follows:

1. REFERENCE TO EXCHANGE AGREEMENT; DEFINITIONS; CERTAIN RULES OF CONSTRUCTION.
Reference is made to the Securities Exchange Agreement dated as of the date
hereof, as from time to time in effect (the "EXCHANGE AGREEMENT"), among iBasis,
Inc., a Delaware corporation (the "COMPANY"), iBasis Global, Inc., a Delaware
corporation ("iBASIS GLOBAL", and collectively with the Company, the
"BORROWER"), the Guarantor, the Symphony Funds and the Collateral Agent.
Capitalized terms defined in the Exchange Agreement and not otherwise defined
herein are used herein with the meanings so defined. Certain other capitalized
terms are used in this Agreement as specifically defined below in this Section
1. Except as the context otherwise explicitly requires, (a) the capitalized term
"Section" refers to sections of this Agreement, (b) the capitalized term
"Exhibit" refers to exhibits to this Agreement, (c) references to a particular
Section shall include all subsections thereof, (d) the word "including" shall be
construed as "including without limitation", (e) references to a particular
statute or regulation include all rules and regulations thereunder and any
successor statute, regulation or rules, in each case as from time to time in
effect and (f) references to a particular Person include such Person's
successors and assigns to the extent not prohibited by this Agreement and the
other Credit Documents. References to "the date hereof" mean the date first set
forth above.

         1.1. "AGREEMENT" means this Guarantee as from time to time in effect.

         1.2. "OBLIGORS" means the Borrower, the Guarantor and the Subsidiaries
of the Borrower party hereto as guarantors from time to time.

2. GUARANTEE.

         2.1. GUARANTEE OF OBLIGATIONS. The Guarantor unconditionally guarantees
that the Obligations will be performed and paid in full in cash when due and
payable, whether at the stated or accelerated maturity thereof or otherwise,
this guarantee being a guarantee of payment and not of collectability and being
absolute and in no way conditional or contingent. In the event any part of the
Obligations shall not have been so paid in full when due and payable, the
Guarantor will, immediately upon notice by the Collateral Agent or, without
notice, immediately upon the occurrence of a Bankruptcy Default, pay or cause to
be paid to the Collateral Agent for the account of each Holder in accordance
with the Holder's respective Percentage Interests


                                       1


<PAGE>


therein the amount of such Obligations which are then due and payable and
unpaid. The obligations of the Guarantor hereunder shall not be affected by the
invalidity, unenforceability or irrecoverability of any of the Obligations as
against the Borrower, any other Obligor, any other guarantor thereof or any
other Person. For purposes hereof, the Obligations shall be due and payable when
and as the same shall be due and payable under the terms of the Exchange
Agreement or any other Credit Document notwithstanding the fact that the
collection or enforcement thereof may be stayed or enjoined under the Bankruptcy
Code or other applicable law.

         2.2. CONTINUING OBLIGATION. The Guarantor acknowledges that the
Symphony Funds have entered into the Exchange Agreement (and, to the extent that
the Symphony Funds, any Holder or holder of any interest in the Obligations or
the Collateral Agent may enter into any future Credit Document, will have
entered into such agreement) in reliance on this Section 2 being a continuing
irrevocable agreement, and the Guarantor agrees that its guarantee may not be
revoked in whole or in part. The obligations of the Guarantor hereunder shall
terminate when all of the Obligations have been indefeasibly paid in full in
cash and discharged; PROVIDED, HOWEVER, that if a claim is made upon the Holders
at any time for repayment or recovery of any amounts or any property received by
the Holders from any source on account of any of the Obligations and the Holders
repay or return any amounts or property so received (including interest thereon
to the extent required to be paid by the Holders), then the Guarantor shall
remain liable under this Agreement for the amounts so repaid or property so
returned or the amounts for which the Holders become liable (such amounts being
deemed part of the Obligations) to the same extent as if such amounts or
property had never been received by the Holders, notwithstanding any termination
hereof or the cancellation of any instrument or agreement evidencing any of the
Obligations. Not later than five days after receipt of notice from the
Collateral Agent, the Guarantor shall pay to the Collateral Agent an amount
equal to the amount of such repayment or return for which the Holders have so
become liable. Payments hereunder by the Guarantor may be required by the
Collateral Agent on any number of occasions.

         2.3. WAIVERS WITH RESPECT TO OBLIGATIONS. Except to the extent
expressly required by the Exchange Agreement or any other Credit Document, the
Guarantor waives, to the fullest extent permitted by the provisions of
applicable law, all of the following (including all defenses, counterclaims and
other rights of any nature based upon any of the following):

                  (a) presentment, demand for payment and protest of nonpayment
         of any of the Obligations, and notice of protest, dishonor or
         nonperformance;

                  (b) notice of acceptance of this guarantee and notice that
         credit has been extended in reliance on the Guarantor's guarantee of
         the Obligations;

                  (c) notice of any Event of Default or of any inability to
         enforce performance of the obligations of the Borrower or any other
         Person with respect to any Credit Document or notice of any
         acceleration of maturity of any Obligations;


                                       2


<PAGE>


                  (d) demand for performance or observance of, and any
         enforcement of any provision of the Exchange Agreement, the Obligations
         or any other Credit Document or any pursuit or exhaustion of rights or
         remedies with respect to any Credit Security or against the Borrower or
         any other Person in respect of the Obligations or any requirement of
         diligence or promptness on the part of the Collateral Agent or the
         Holders in connection with any of the foregoing;

                  (e) any act or omission on the part of the Collateral Agent or
         the Holders which may impair or prejudice the rights of the Guarantor,
         including rights to obtain subrogation, exoneration, contribution,
         indemnification or any other reimbursement from the Borrower or any
         other Person, or otherwise operate as a deemed release or discharge;

                  (f) failure or delay to perfect or continue the perfection of
         any security interest in any Credit Security or any other action which
         harms or impairs the value of, or any failure to preserve or protect
         the value of, any Credit Security;

                  (g) any statute of limitations or any statute or rule of law
         which provides that the obligation of a surety must be neither larger
         in amount nor in other respects more burdensome than the obligation of
         the principal;

                  (h) any "single action" or "anti-deficiency" law which would
         otherwise prevent the Holders from bringing any action, including any
         claim for a deficiency, against the Guarantor before or after the
         Collateral Agent's or the Holders' commencement or completion of any
         foreclosure action, whether judicially, by exercise of power of sale or
         otherwise, or any other law which would otherwise require any election
         of remedies by the Collateral Agent or the Holders;

                  (i) all demands and notices of every kind with respect to the
         foregoing; and

                  (j) to the extent not referred to above, all defenses (other
         than payment) which the Borrower may now or hereafter have to the
         payment of the Obligations, together with all suretyship defenses,
         which could otherwise be asserted by the Guarantor.

Each Guarantor represents that it has obtained the advice of counsel as to the
extent to which suretyship and other defenses may be available to it with
respect to its obligations hereunder in the absence of the waivers contained in
this Section 2.3.

         No delay or omission on the part of the Collateral Agent or the Holders
in exercising any right under any other Credit Document or under any other
guarantee of the Obligations or with respect to the Credit Security shall
operate as a waiver or relinquishment of such right. Absent gross negligence or
willful misconduct on the Collateral Agent's or Holders' part, no action which
the Collateral Agent or the Holders or the Borrower or any other Obligor may
take or refrain from taking with respect to the Obligations shall affect the
provisions of this Agreement or the obligations of the Guarantor hereunder. None
of the Holders' or the Collateral Agent's


                                       3


<PAGE>


rights shall at any time in any way be prejudiced or impaired by any act or
failure to act on the part of the Borrower or any other Obligor, or by any
noncompliance by the Borrower or any other Obligor with any Credit Document,
regardless of any knowledge thereof which the Collateral Agent or the Holders
may have or otherwise be charged with.

         2.4. HOLDERS' POWER TO WAIVE, ETC. The Guarantor grants to the
Collateral Agent and the Holders full power in their discretion, without notice
to or consent of the Guarantor, such notice and consent being expressly waived
to the fullest extent permitted by applicable law, and without in any way
affecting the liability of the Guarantor under its guarantee hereunder:

                  (a) To waive compliance with, and any Default under, and to
         consent to any amendment to or modification or termination of any
         provision of, or to give any waiver in respect of, the Exchange
         Agreement, any other Credit Document, the Credit Security, the
         Obligations or any guarantee thereof (each as from time to time in
         effect);

                  (b) To grant any extensions of the Obligations (for any
         duration), and any other indulgence with respect thereto, and to effect
         any total or partial release (by operation of law or otherwise),
         discharge, compromise or settlement with respect to the obligations of
         the Obligors or any other Person in respect of the Obligations, whether
         or not rights against the Guarantor under this Agreement are reserved
         in connection therewith;

                  (c) To take security in any form for the Obligations, and to
         consent to the addition to or the substitution, exchange, release or
         other disposition of, or to deal in any other manner with, any part of
         any property contained in the Credit Security whether or not the
         property, if any, received upon the exercise of such power shall be of
         a character or value the same as or different from the character or
         value of any property disposed of, and to obtain or release any present
         or future guarantees of the Obligations and to proceed against any of
         the Credit Security or such guarantees in any order;

                  (d) To collect or liquidate or realize upon any of the
         Obligations or the Credit Security in a commercially reasonable manner
         or to refrain from collecting or liquidating or realizing upon any of
         the Obligations or the Credit Security; and

                  (e) To extend credit under the Exchange Agreement, any other
         Credit Document or otherwise in such amount as the Holders may
         determine, including increasing the amount of credit and the interest
         rate and fees with respect thereto, even though the condition of the
         Obligors (financial or otherwise, on an individual or Consolidated
         basis) may have deteriorated since the date hereof.

         2.5. INFORMATION REGARDING THE BORROWER, ETC. The Guarantor has made
such investigation as it deems desirable of the risks undertaken by them in
entering into this Agreement and are fully satisfied that it understands all
such risks. The Guarantor waives any obligation which may now or hereafter exist
on the part of the Collateral Agent or the Holders to inform it of the risks
being undertaken by entering into this Agreement or of any changes in such


                                       4


<PAGE>


risks and, from and after the date hereof, the Guarantor undertakes to keep
themselves informed of such risks and any changes therein. The Guarantor
expressly waives any duty which may now or hereafter exist on the part of the
Collateral Agent or the Holders to disclose to such Guarantor any matter related
to the business, operations, character, collateral, credit, condition (financial
or otherwise), income or prospects of the Borrower and its Affiliates or their
properties or management, whether now or hereafter known by the Collateral Agent
or the Holders. The Guarantor represents, warrants and agrees that it assumes
sole responsibility for obtaining from the Borrower all information concerning
the Exchange Agreement and all other Credit Documents and all other information
as to the Borrower and its Affiliates or their properties or management as the
Guarantor deems necessary or desirable.

         2.6. CERTAIN GUARANTOR REPRESENTATIONS. Each Guarantor represents that:

                  (a) it is in its best interest and in pursuit of the purposes
         for which it was organized as an integral part of the business
         conducted and proposed to be conducted by the Borrower and its
         Subsidiaries, and reasonably necessary and convenient in connection
         with the conduct of the business conducted and proposed to be conducted
         by them, to induce the Symphony Funds to enter into the Exchange
         Agreement and to extend credit to the Guarantor by making the Guarantee
         contemplated by this Section 2;

                  (b) the credit available under the Exchange Agreement will
         directly or indirectly inure to its benefit;

                  (c) by virtue of the foregoing it is receiving at least
         reasonably equivalent value from the Symphony Funds for its Guarantee;

                  (d) it will not be rendered insolvent as a result of entering
         into this Agreement;

                  (e) after giving effect to the transactions contemplated by
         this Agreement, it will have assets having a fair saleable value in
         excess of the amount required to pay its probable liability on its
         existing debts as such debts become absolute and matured;

                  (f) it has, and will have, access to adequate capital for the
         conduct of its business;

                  (g) it has the ability to pay its debts from time to time
         incurred in connection therewith as such debts mature; and

                  (h) it has been advised by the Collateral Agent that the
         Symphony Funds are unwilling to enter into the Exchange Agreement
         unless the Guarantee contemplated by this Section 2 is given by it.

         2.7. SUBROGATION. The Guarantor agrees that, until the Obligations are
indefeasibly paid in full, it will not exercise any right of reimbursement,
subrogation, contribution, offset or


                                       5


<PAGE>


other claims against the Borrower or any other Obligor arising by contract or
operation of law in connection with any payment made or required to be made by
the Guarantor under this Agreement. After the payment in full of the
Obligations, the Guarantor shall be entitled to exercise against the Borrower
and the other Obligors all such rights of reimbursement, subrogation,
contribution and offset, and all such other claims, to the fullest extent
permitted by law.

         2.8. SUBORDINATION. The Guarantor covenants and agrees that all
Indebtedness, claims and liabilities now or hereafter owing by the Borrower or
any other Obligor to the Guarantor whether arising hereunder or otherwise are
subordinated to the prior payment in full of the Obligations and are so
subordinated as a claim against such Obligor or any of its assets, whether such
claim be in the ordinary course of business or in the event of voluntary or
involuntary liquidation, dissolution, insolvency or bankruptcy, so that no
payment with respect to any such Indebtedness, claim or liability will be made
or received while any Event of Default exists.

         2.9. FUTURE SUBSIDIARIES; FURTHER ASSURANCES. The Guarantor will from
time to time cause (a) any present Wholly Owned Subsidiary that is not a
Guarantor within 30 days after notice from the Collateral Agent or (b) any
future Wholly Owned Subsidiary within 30 days after any such Person becomes a
Wholly Owned Subsidiary, to join this Agreement as a Guarantor pursuant to a
joinder agreement in form and substance satisfactory to the Collateral Agent;
PROVIDED, HOWEVER, that in the event such a Wholly Owned Subsidiary is
prohibited by any valid law, statute, rule or regulation from guaranteeing the
Obligations, or if such a guarantee by any foreign Subsidiary would result in a
repatriation of a material amount of foreign earnings under the Code (including
the "deemed dividend" provisions of section 956 of the Code), (i) such guarantee
will be limited to the extent necessary to comply with such prohibition or to
prevent such repatriation of foreign earnings or (ii) if such limitation on the
guaranteed amount is not sufficient to avoid such prohibition or repatriation,
the Borrower and its other Subsidiaries will pledge the stock of such Wholly
Owned Subsidiary (or as much of such stock as may be pledged without resulting
in such a repatriation) to the Collateral Agent to secure the Obligations
pursuant to a pledge agreement in form and substance satisfactory to the
Collateral Agent. The Guarantor will, promptly upon the request of the
Collateral Agent from time to time, execute, acknowledge and deliver, and file
and record, all such instruments, and take all such action, as the Collateral
Agent deems necessary or advisable to carry out the intent and purpose of this
Section 2.

3. REPRESENTATIONS AND WARRANTIES. In order to induce the Symphony Funds to
extend credit under the Exchange Agreement, the Guarantor represents and
warrants that:

         3.1. ORGANIZATION AND BUSINESS. The Guarantor is a duly organized and
validly existing corporation, in good standing under the laws of The
Commonwealth of Massachusetts, as applicable, with all power and authority,
corporate or otherwise, necessary (a) to enter into and perform this Agreement
and each other Credit Document to which it is a party and (b) to own its
properties and carry on the business now conducted or proposed to be conducted
by it.


                                       6


<PAGE>


Certified copies of the Charter and By-laws of the Guarantor have been
previously delivered to the Collateral Agent and are correct and complete.

         3.2. AUTHORIZATION AND ENFORCEABILITY. The Guarantor has taken all
corporate action required to execute, deliver and perform this Agreement and
each other Credit Document to which it is a party. Each of this Agreement and
each other Credit Document to which the Guarantor is party constitutes the
legal, valid and binding obligation of the Guarantor, enforceable against the
Guarantor in accordance with its terms.

         3.3. NO LEGAL OBSTACLE TO AGREEMENTS. Neither the execution, delivery
and performance of this Agreement or any other Credit Document to which it is
party, nor the consummation of any transaction referred to in or contemplated by
this Agreement or any other Credit Document, has constituted or resulted, or
will constitute or result, in:

                  (a) Any breach or termination of the provisions of any
         agreement, instrument, deed or lease to which the Guarantor is a party
         or by which it is bound, or of the Charter or By-laws of the Guarantor;
         or

                  (b) The violation of any law, statute, judgment, decree or
         governmental order, rule or regulation applicable to the Guarantor.

No approval, authorization or other action by, or declaration to or filing with,
any governmental or administrative authority or any other Person is required to
be obtained or made by the Guarantor in connection with the execution, delivery
and performance of this Agreement or any other Credit Document to which it is
party or the transactions contemplated hereby or thereby.

         3.4. LITIGATION. Except as disclosed in the SEC Reports, no litigation,
at law or in equity, or any proceeding before any court, board or other
governmental or administrative agency or any arbitrator is pending or, to the
knowledge of the Guarantor, threatened which may involve any material risk of
any final judgment, order or liability which, after giving effect to any
applicable insurance, has resulted, or creates a material risk of resulting, in
any material adverse change in the Guarantor's business, assets, financial
condition, income or prospects or which seeks to enjoin the consummation, or
which questions the validity, of any of the transactions contemplated by this
Agreement or any other Credit Document. No judgment, decree or order of any
court, board or other governmental or administrative agency or any arbitrator
has been issued against or binds the Guarantor which has resulted, or creates a
material risk of resulting, in any material adverse change in the Guarantor's
business, assets, financial condition, income or prospects.


                                       7


<PAGE>


4. SUCCESSORS AND ASSIGNS. The provisions of this Agreement shall inure to the
benefit of the Holders and their successors and assigns and shall be binding
upon the Guarantor and its respective successors and assigns. The Guarantor may
not assign its rights or obligations under this Agreement without the written
consent of the Collateral Agent.

5. NOTICES. Any notice or other communication in connection with this Agreement
shall be deemed to be given if given in writing (including telex, telecopy or
similar teletransmission) addressed as provided below (or to the addressee at
such other address as the addressee shall have specified by notice actually
received by the addressor), and if either (a) actually delivered in fully
legible form to such address (evidenced in the case of a telex by receipt of the
correct answerback) or (b) in the case of a letter, five business days shall
have elapsed after the same shall have been deposited in the United States
mails, with first-class postage prepaid and registered or certified.

         If to the Guarantor, to it in care of iBasis, Inc., 20 Second Avenue,
Burlington, MA 01803, to the attention of its chief financial officer.

         If to the Collateral Agent, to it at its address specified in or
pursuant to section 13 of the Exchange Agreement.

6. REIMBURSEMENT OF EXPENSES. Subject to the terms of the Fee Letter (as such
term is defined in the Exchange Agreement) and the Exchange Agreement, the
Guarantor shall promptly pay on demand all reasonable expenses of the Collateral
Agent and the Holders (including reasonable attorney fees and expenses) in
connection with the preparation of this Agreement, operations hereunder and
enforcement and collection hereof, whether before or after bankruptcy or similar
proceedings (and whether or not allowed as a claim therein).

7. VENUE; SERVICE OF PROCESS. Each Guarantor:

                  (a) Irrevocably submits to the nonexclusive jurisdiction of
         the state courts of the Commonwealth of Massachusetts and to the
         nonexclusive jurisdiction of the United States District Court for the
         District of Massachusetts for the purpose of any suit, action or other
         proceeding arising out of or based upon this Agreement or any other
         Credit Document or the subject matter hereof or thereof;

                  (b) Waives to the extent not prohibited by applicable law, and
         agrees not to assert, by way of motion, as a defense or otherwise, in
         any such proceeding brought in any of the above-named courts, any claim
         that it is not subject personally to the jurisdiction of such court,
         that its property is exempt or immune from attachment or execution,
         that such proceeding is brought in an inconvenient forum, that the
         venue of any such proceeding is improper, or that this Agreement or any
         other Credit Document, or the subject matter hereof or thereof, may not
         be enforced in or by such court;

                  (c) Consents to service of process in any such proceeding in
         any manner permitted by law and agrees that service of process by
         registered or certified mail, return


                                       8


<PAGE>


         receipt requested, at its address specified in or pursuant to Section 5
         is reasonably calculated to give actual notice; and

                  (d) Waives to the extent not prohibited by applicable law that
         cannot be waived any right it may have to claim or recover in any such
         proceeding any special, exemplary, punitive or consequential damages.

8. WAIVER OF JURY TRIAL. TO THE EXTENT NOT PROHIBITED BY APPLICABLE LAW WHICH
CANNOT BE WAIVED, EACH OF THE COLLATERAL AGENT AND EACH GUARANTOR WAIVES, AND
COVENANTS THAT IT WILL NOT ASSERT (WHETHER AS PLAINTIFF, DEFENDANT OR
OTHERWISE), ANY RIGHT TO TRIAL BY JURY IN ANY FORUM IN RESPECT OF ANY ISSUE,
CLAIM, DEMAND OR ACTION ARISING OUT OF OR BASED UPON THIS AGREEMENT, THE
SECURITIES EXCHANGE AGREEMENT OR ANY OTHER CREDIT DOCUMENT OR THE SUBJECT MATTER
HEREOF OR THEREOF OR ANY OBLIGATION OR IN ANY WAY CONNECTED WITH THE DEALINGS OF
THE COLLATERAL AGENT OR THE GUARANTOR IN CONNECTION WITH ANY OF THE ABOVE, IN
EACH CASE WHETHER NOW EXISTING OR HEREAFTER ARISING AND WHETHER IN CONTRACT OR
TORT OR OTHERWISE. Each Guarantor and the Collateral Agent acknowledges that the
provisions of this Section constitute a material inducement upon which each
party has relied, is relying and will rely in entering into the Exchange
Agreement and any other Credit Document, and that it has reviewed the provisions
of this Section with its counsel. The Collateral Agent or the Guarantor may file
an original counterpart or a copy of this Section with any court as written
evidence of the consent of the Collateral Agent and the Guarantor to the waiver
of the right to trial by jury.

9. GENERAL. All covenants, agreements, representations and warranties made in
this Agreement or any other Credit Document or in certificates delivered
pursuant hereto or thereto shall be deemed to have been relied on by each
Symphony Fund, notwithstanding any investigation made by the Collateral Agent on
its behalf, and shall survive the execution and delivery to the Symphony Funds
and the Holders hereof and thereof. The invalidity or unenforceability of any
provision hereof shall not affect the validity or enforceability of any other
provision hereof, and any invalid or unenforceable provision shall be modified
so as to be enforced to the maximum extent of its validity or enforceability.
The headings in this Agreement are for convenience of reference only and shall
not limit, alter or otherwise affect the meaning hereof. This Agreement and the
other Credit Documents constitute the entire understanding of the parties with
respect to the subject matter hereof and thereof and supersede all prior and
current understandings and agreements, whether written or oral. This Agreement
is a Credit Document and may be executed in any number of counterparts, which
together shall constitute one instrument. This Agreement shall be governed by,
and shall be construed and enforced in accordance with, the laws of The
Commonwealth of Massachusetts, without regard to the principles of conflicts of
laws. Each beneficial holder of the Obligations shall be entitled to the
benefits of a Holder under this Agreement.


                                       9


<PAGE>


         Each of the undersigned has caused this Agreement to be executed and
delivered by its duly authorized officer as an agreement under seal as of the
date first written above.




                               iBASIS SECURITIES CORPORATION



                               By /s/ Gordon VanderBrug
                                  ---------------------------------------------
                                  Title: Executive Vice President




                               U.S. BANK NATIONAL ASSOCIATION,
                               as Collateral Agent under the Exchange Agreement



                               By /s/ John A. Brennan
                                  ---------------------------------------------
                                  Title: Trust Officer


                                       10