EXHIBIT 10.3

IBASIS, INC.

AND

THE HOLDERS

LISTED ON ANNEX A HERETO

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REGISTRATION RIGHTS AGREEMENT

DATED AS OF MAY 29, 2003

TABLE OF CONTENTS

Page

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Section 1.   Registration Rights   1  

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Section 2.   Definitions   10  

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Section 3.   Notices   12  

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Section 4.   Successors   13  

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Section 5.   Governing Law; Submission to Jurisdiction: Waiver of Jury Trial  
13  

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Section 6.   Exercise of Rights and Remedies   13  

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Section 7.   Benefits of This Agreement   13  

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Section 8.   Counterparts   14  

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Section 9.   Additional Parties   14  

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        REGISTRATION RIGHTS AGREEMENT (this “Agreement”), dated as of May 29,
2003, between (i) iBasis, Inc., a Delaware corporation (the “Company”), (ii) the
persons listed on Annex A hereto (the “Initial Holders”), and (iii) each other
Person who becomes a party to this Agreement pursuant to Section 9 hereof (such
other Persons together with the Initial Holders, the “Holders”). Capitalized
terms used herein and not otherwise defined shall have the meaning ascribed to
such terms in Section 2 hereof.

        WHEREAS, the Company proposes to issue to the Holders (i) the
Commonwealth Warrants, which in the aggregate initially entitle the Holders to
purchase, 1,116,605 shares of Common Stock of the Company on May 29, 2003, and
(ii) the Commonwealth Notes, in the aggregate principal amount of $6,100,000, in
each case pursuant to a Joinder Agreement (dated as of May 29, 2003, by and
among the Company, iBasis Global, Inc., the Holders and U.S. Bank National
Association, as Collateral Agent (the “Joinder Agreement”)) to that certain
Securities Exchange Agreement, dated as of February 21, 2003, among the Company,
iBasis Global, Inc., the guarantors named therein, JMG Triton Offshore Limited
Fund CITCO and U.S. Bank National Association, as Collateral Agent (as amended,
supplemented and otherwise modified from time to time, the “Securities Exchange
Agreement”).

        WHEREAS, the Company and the Holders desire to enter into this Agreement
to provide the Holders registration rights with respect to the Commonwealth
Warrants and the Commonwealth Notes.

        WHEREAS, the Commonwealth Warrants and the Commonwealth Notes shall have
registration rights as provided herein.

        NOW, THEREFORE, in consideration of the premises and the mutual
agreements herein set forth, the parties hereto agree as follows:

        Section 1.  Registration Rights. The Company will perform and comply,
and cause each of its Subsidiaries to perform and comply, with each of the
following provisions as are applicable to it. Each Holder will perform and
comply with each of the following provisions as are applicable to such holder.

                1.1   Demand Registration Rights.

                        1.1.1.       General. At any time after July 30, 2003,
one or more holders of (i) the Commonwealth Warrants who at such time hold
greater than 20% of the Commonwealth Warrants (determined based on an
as-exercised basis), or (ii) the Commonwealth Notes who at such time hold
greater than 20% of the aggregate principal amount of the Commonwealth Notes (in
either case, the “Initiating Holders”), may request, by written notice to the
Company, that the Company effect the registration under the Securities Act for a
Public Offering of all or a specified part of the Registrable Securities held by
such Initiating Holders. Such notice shall specify the intended method or
methods of distribution and the type of Registrable Securities intended to be so
registered. Promptly after receipt of such notice from the Initiating Holders,
the Company shall give notice of such requested registration to all other
holders of Registrable Securities in accordance with Section 1.2. The Company
will then use its reasonable best efforts to effect the registration under the
Securities Act of the Registrable Securities which the Company has been
requested to register by such Initiating Holders, together with all other
Registrable Securities which the Company has been requested to register pursuant
to Section 1.2 or otherwise by notice delivered to the Company within 20 days
after the Company has given the required notice of such requested registration
(which request shall specify the intended method of disposition of such
Registrable Securities), all to the extent requisite to permit the disposition
(in accordance with the intended methods thereof as aforesaid) of the
Registrable Securities which the Company has been so requested to register;
provided, however, that the Company shall not be obligated to take any action to
effect any such registration pursuant to this Section 1.1.1 within 180 days
immediately following the effective date of any registration statement
pertaining to an underwritten public offering of securities of the Company for
its own account (other than a Rule 145 Transaction, or a registration relating
solely to employee benefit plans).

                        1.1.2.       Form; Limitations. Except as otherwise
provided above, each registration requested pursuant to Section 1.1.1 shall be
effected by the filing of a registration statement on Form S-1 (or any other
form which includes substantially the same information as would be required to
be included in a registration statement on such form as currently constituted),
unless the use of a different form has been agreed to in writing by holders of
at least a majority of the Registrable Securities to be included in the proposed
registration statement in question (the “Majority Participating Holders”) or the
Company is then eligible to use Form S-3 for a Public Offering by the Majority
Participating Holders of their Registrable Securities; provided, however, that
the Company shall not be required to effect any registration requested pursuant
to Section 1.1.1 on any form other than Form S-3 (or any successor form) if the
Company has previously effected three or more registrations of Registrable
Securities under Section 1.1.1 on any form other than Form S-3 (or any successor
form). No registration of Registrable Securities which shall not have become and
remained effective in accordance with this Section 1.1 shall be included in the
calculation of the number of registrations contemplated by this Section 1.1.2
(unless the Initiating Holders withdraw their request for such registration,
other than (i) if such withdrawal is a result of information concerning the
business or financial condition of the Company which is made known after the
date on which such registration was requested or (ii) if the Initiating Holders
pay all the expenses of such registration otherwise payable by the Company
pursuant to Section 1.1.3). If at the time of any request to register
Registrable Securities pursuant to this Section 1.1.1, the Company is engaged or
has plans to engage in a registered public offering or is engaged in any other
activity which, in the good faith determination of the Board, would be adversely
affected by the requested registration, then the Company may at its option
direct that such request be delayed for a period not in excess of 30 days from
the date of such request, such right to delay a request to be exercised by the
Company not more than once in any 12-month period. In the event that, in the
judgment of the Company, it is advisable to suspend use of a prospectus included
in a registration statement due to pending material developments or other events
that have not yet been publicly disclosed and as to which the Company believes
public disclosure would be detrimental to the Company, the Company shall notify
all selling stockholders to such effect, and, upon receipt of such notice, each
such selling stockholder shall immediately discontinue any sales of Registrable
Securities pursuant to such registration statement until such selling
stockholder has received copies of a supplemented or amended prospectus or until
such selling stockholder is advised in writing by the Company that the then
current prospectus may be used and has received copies of any additional or
supplemental filings that are incorporated or deemed incorporated by reference
in such prospectus. Notwithstanding anything to the contrary herein, the Company
shall not exercise its rights under the foregoing sentence to suspend sales of
Registrable Securities for a period in excess of 30 days consecutively or 60
days in any 365-day period.

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                        1.1.3.       Payment of Expenses. The Company shall pay
all reasonable expenses of holders of Registrable Securities incurred in
connection with the first three registrations of Registrable Securities
requested pursuant to this Section 1.1 (including the reasonable fees and
expenses of a single legal counsel representing all selling stockholders), other
than underwriting discounts and commissions, if any, and applicable transfer
taxes, if any.

                        1.1.4.       Additional Procedures. In the case of a
registration pursuant to Section 1.1.2, whenever the Initiating Holders shall
request that such registration shall be effected pursuant to an underwritten
offering, the Company shall include such information in the written notices to
holders of Registrable Securities referred to in Section 1.1.2. In such event,
the right of any holder of Registrable Securities to have securities owned by
such holder included in such registration pursuant to this Section 1.1.1 shall
be conditioned upon such holder’s participation in such underwriting and the
inclusion of such holder’s Registrable Securities in the underwriting (unless
otherwise mutually agreed upon by the Majority Participating Holders and such
holder). If requested by the underwriters of such registration, the Company,
together with the holders of Registrable Securities proposing to distribute
their securities through such underwriting, will enter into an underwriting
agreement with such underwriters for such offering containing such
representations and warranties by the Company and such holders and such other
terms and provisions as are customarily contained in underwriting agreements
with respect to secondary distributions, including, without limitation,
customary indemnity and contribution provisions (subject, in each case, to the
limitations on such liabilities set forth in this Agreement).

                1.2   Piggyback Registration Rights.

                        1.2.1        General. Each time the Company proposes to
register any of its securities under the Securities Act on a form which would
permit registration of Registrable Securities for sale to the public, for its
own account and/or for the account of any holders of Registrable Securities or
Affiliate of a holder of Registrable Securities (pursuant to Section 1.1 or
otherwise), for sale in a Public Offering, the Company will give notice to all
holders of Registrable Securities of its intention to do so. Any such holder
may, by written response delivered to the Company within 20 days after the
effectiveness of such notice, request that all or a specified part of the
Registrable Securities held by such holder of the same type of Registrable
Securities to be so registered by the Company be included in such registration.
The Company thereupon will use its reasonable efforts to cause to be included in
such registration under the Securities Act all such Registrable Securities which
the Company has been so requested to register by such holders, to the extent
required to permit the disposition (in accordance with the methods to be used by
the Company or other holders of its securities in such Public Offering) of the
Registrable Securities to be so registered. No registration of Registrable
Securities effected under this Section 1.2 shall relieve the Company of any of
its obligations to effect registrations of Registrable Securities pursuant to
Section 1.1. The Company may withdraw or suspend any registration covered by
this Section 1.2 at any time (subject, in the case of any registration also
covered by Section 1.1, to any limitations set forth therein).

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                        1.2.2.       Excluded Transactions. The Company shall
not be obligated to effect any registration of Registrable Securities under this
Section 1.2 incidental to the registration of any of its securities in
connection with:

    (a)        Any Public Offering relating to employee benefit plans or
dividend reinvestment plans; or

    (b)        Any Public Offering relating to the acquisition or merger after
the date hereof by the Company or any of its Subsidiaries of or with any other
businesses.

                        1.2.3        Payment of Expenses. The Company shall pay
all reasonable fees and expenses of a single legal counsel representing any and
all holders of Registrable Securities incurred in connection with the first
three registrations of Registrable Securities requested pursuant to this
Section 1.2.

                        1.2.4.        Additional Procedures. Holders of
Registrable Securities participating in any Public Offering pursuant to this
Section 1.2 shall take all such actions and execute all such documents and
instruments that are reasonably requested by the Company to effect the sale of
their Registrable Securities in such Public Offering, including, without
limitation, being parties to the underwriting agreement entered into by the
Company and any other selling securityholders in connection therewith and being
liable in respect of the representations and warranties by, and the other
agreements (including without limitation customary selling securityholder
representations, warranties, indemnifications and “lock-up” agreements) for the
benefit of the underwriters; provided, however, that (a) with respect to
individual representations, warranties, indemnities and agreements of selling
holders of Registrable Securities in such Public Offering, the aggregate amount
of such liability shall not exceed such holder’s net proceeds from such offering
and (b) to the extent selling holders of Registrable Securities give further
representations, warranties and indemnities, then with respect to all other
representations, warranties and indemnities of sellers of securities in such
Public Offering, the aggregate amount of such liability shall not exceed the
lesser of (i) such holder’s pro rata portion of any such liability, in
accordance with such holder’s portion of the total number of Registrable
Securities included in the offering or (ii) such holder’s net proceeds from such
offering.

                1.3.       Certain Other Provisions.

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                        1.3.1.        Underwriter’s Cutback. In connection with
any registration of Registrable Securities, the underwriter may determine that
marketing factors (including, without limitation, an adverse effect on the per
share offering price) require a limitation of the number of Registrable
Securities to be underwritten. Notwithstanding any contrary provision of Section
1.1 or Section 1.2, and subject to the terms of this Section 1.3.1, the
underwriter may limit the number of securities which would otherwise be included
in such registration by excluding any or all Registrable Securities from such
registration (it being understood that the number of securities which the
Company seeks to have registered in such registration shall not be subject to
exclusion, in whole or in part, under this Section 1.3.1). Upon receipt of
notice from the underwriter of the need to reduce the number of securities to be
included in the registration, the Company shall advise all holders of the
Company’s securities that would otherwise be registered and underwritten
pursuant hereto, and the number of such securities, including Registrable
Securities, that may be included in the registration shall be allocated in the
following manner, unless the underwriter shall determine that marketing factors
require a different allocation: (i) securities, other than Registrable
Securities, requested to be included in such registration by securityholders
shall be excluded unless (x) the Company has, prior to the date hereof, or after
the date hereof with the consent of the Majority Holders, granted registration
rights which are to be treated on an equal basis with Registrable Securities for
the purpose of the exercise of the underwriter cutback (with it being
specifically agreed and acknowledged that the registration rights granted to
Silicon Valley Bank pursuant to the Registration Rights Agreement, dated on or
around December 30, 2002, between the Company and Silicon Valley Bank are to be
treated on an equal basis with Registrable Securities for the purposes of this
underwriting cutback), or (y) such securities are issued in connection with a
Permitted Exchange (as defined in the Securities Exchange Agreement), with it
being specifically acknowledged that the registration rights granted to the
holders of any such securities are to be treated on an equal basis with
Registrable Securities for purposes of this underwriting cutback; and (ii) if
further limitation on the number of securities to be included in the offering is
required, the number of Registrable Securities and other securities that may be
included in such registration shall be allocated among holders thereof in
proportion, as nearly as practicable, to the respective amounts of securities
which each securityholder requested be registered in such registration. No
securities excluded from the underwriting by reason of the underwriter’s
marketing limitation shall be included in such registration. Upon delivery of a
written request that Registrable Securities be included in an underwritten
offering pursuant to Section 1.1.1 or Section 1.2.1, the holder thereof may not
thereafter (i) elect to withdraw therefrom without the written consent the
Company and the Majority Participating Holders, or (ii) exchange, convert or
exercise, as the case may be, such Registrable Securities.

                        1.3.2.        Registration Procedures. If and in each
case when the Company is required to use its reasonable best efforts to effect a
registration of any Registrable Securities as provided in Section 1.1 or Section
1.2, the Company shall take appropriate and customary actions in furtherance
thereof, including, without limitation:

    (a)        promptly filing with the Commission a registration statement and
using best efforts to cause such registration statement to become effective;

    (b)        preparing and filing with the Commission such amendments and
supplements to such registration statements as may be required to comply with
the Securities Act and to keep such registration statement effective for a
period not to exceed 270 days from the date of effectiveness or such earlier
time as the Registrable Securities covered by such registration statement shall
have been disposed of in accordance with the intended method of distribution
therefor or the expiration of the time when a prospectus relating to such
registration is required to be delivered under the Securities Act; provided,
however, that if the registration is effected by the filing of a registration
statement on Form S-3, then the Company shall keep such registration statement
effective for a period not to exceed 3 years from the date of effectiveness or
such earlier time as the Registrable Securities covered by such registration
statement have been disposed of in accordance with the intended method of
distribution therefore, the expiration of the time when a prospectus relating to
such registration is required to be delivered under the Securities Act, or such
registration statement no longer covers Registrable Securities;

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    (c)        using its best efforts to register or qualify such Registrable
Securities under the state securities or “blue sky” laws of such jurisdictions
as the sellers shall reasonably request; provided, however, that the Company
shall not be obligated to file any general consent to service of process or to
qualify as a foreign corporation in any jurisdiction in which it is not so
qualified or to subject itself to taxation in respect of doing business in any
jurisdiction in which it would not otherwise be so subject; and

    (d)        otherwise cooperating reasonably with, and taking such customary
actions as may reasonably be requested by the holders of Registrable Securities
in connection with, such registration.

                        1.3.3.              Selection of Underwriters and
Counsel. The underwriters and legal counsel to be retained in connection with
any Public Offering shall be selected by the Board or, in the case of an
offering following a request therefor under Section 1.1.1, the Initiating
Holders with the consent of the Company (which consent shall not be unreasonably
withheld).

                        1.3.4.              Lock-Up. Without the prior written
consent of the underwriters managing any Public Offering, for a period beginning
seven days immediately preceding and ending on the 90th day following the
effective date of the registration statement used in connection with such
offering, no holder of Commonwealth Warrants (whether or not a selling
securityholder pursuant to such registration statement) representing (on an
as-exercised basis) at least 1% of the outstanding Common Stock shall (a) offer,
pledge, sell, contract to sell, sell any option or contract to purchase,
purchase any option or contract to sell, grant any option, right or warrant to
purchase, lend, or otherwise transfer, directly or indirectly, any shares of
Common Stock or any securities convertible into or exercisable or exchangeable
for such Common Stock or (b) enter into any swap or other arrangement that
transfers to another, in whole or in part, any of the economic consequences of
ownership of Common Stock, whether any such transaction described in clause
(a) or (b) above is to be settled by delivery of such Common Stock or such other
securities, in cash or otherwise; provided, however, that the foregoing
restrictions shall not apply to (i) the sale of Commonwealth Warrants pursuant
to any such registration statement filed in accordance with Section 1.1 or
Section 1.2; (ii) transactions relating to shares of Common Stock or other
securities acquired in open market transactions; or (iii) the exercise,
conversion or exchange of Commonwealth Warrants or conversions of shares of
Common Stock into other classes of Common Stock without change of holder.

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                        1.3.5.              Future Registration Rights. If,
after the date hereof, the Company enters into an agreement or other commitment
with any other Person that has the effect of establishing registration rights
with respect to the Company’s securities the terms of which are more favorable,
taken as a whole, to such Person than the registration rights established in
favor of the holders of Registrable Securities pursuant to Section 1.1 or
Section 1.2, then the Company will promptly so notify such holders in writing,
and the Company shall, without the necessity of any action on the part of such
holders, extend the benefits of such more favorable terms to such holders as if
such terms were contained in this Agreement, or permit such holders to enter
into such other agreement establishing such rights in lieu of this agreement.

                1.4.       Indemnification and Contribution.

                        1.4.1.              Indemnities of the Company. In the
event of any registration of any Registrable Securities or other debt or equity
securities of the Company or any of its Subsidiaries under the Securities Act
pursuant to Section 1.1, Section 1.2 or otherwise, and in connection with any
registration statement or any other disclosure document produced by or on behalf
of the Company or any of its Subsidiaries including, without limitation, reports
required and other documents filed under the Exchange Act, and other documents
pursuant to which any debt or equity securities of the Company or any of its
Subsidiaries are sold (whether or not for the account of the Company or its
Subsidiaries), the Company will, and hereby does, and will cause each of its
Subsidiaries, jointly and severally, to indemnify and hold harmless each seller
of Registrable Securities, any Person who is or might be deemed to be a
controlling Person of the Company or any of its Subsidiaries within the meaning
of section 15 of the Securities Act or section 20 of the Exchange Act, their
respective direct and indirect partners, advisory board members, directors,
officers, trustees, members and stockholders, and each other Person, if any, who
controls any such seller or any such controlling Person within the meaning of
section 15 of the Securities Act or section 20 of the Exchange Act (each such
person being referred to herein as a “Covered Person”), against any losses,
claims, damages or liabilities (or actions or proceedings in respect thereof),
joint or several, to which such Covered Person may be or become subject under
the Securities Act, the Exchange Act, any other securities or other law of any
jurisdiction, the common law or otherwise, insofar as such losses, claims,
damages or liabilities (or actions or proceedings in respect thereof) arise out
of or are based upon (i) any untrue statement or alleged untrue statement of any
material fact contained or incorporated by reference in any registration
statement under the Securities Act, any preliminary prospectus or final
prospectus included therein, or any related summary prospectus, or any amendment
or supplement thereto, or any document incorporated by reference therein, or any
other such disclosure document (including without limitation reports and other
documents filed under the Exchange Act and any document incorporated by
reference therein) or other document or report, (ii) any omission or alleged
omission to state therein a material fact required to be stated therein or
necessary to make the statements therein not misleading or (iii) any violation
or alleged violation by the Company or any of its subsidiaries of any federal,
state, foreign or common law rule or regulation applicable to the Company or any
of its Subsidiaries and relating to action or inaction in connection with any
such registration, disclosure document or other document or report, and will
reimburse such Covered Person for any legal or any other reasonable expenses
incurred by it in connection with investigating or defending any such loss,
claim, damage, liability, action or proceeding; provided, however, that neither
the Company nor any of its Subsidiaries shall be liable to any Covered Person in
any such case to the extent that any such loss, claim, damage, liability, action
or proceeding arises out of or is based upon an untrue statement or alleged
untrue statement or omission or alleged omission made in such registration
statement, preliminary prospectus, final prospectus, summary prospectus,
amendment or supplement, incorporated document or other such disclosure document
or other document or report, in reliance upon and in conformity with written
information furnished to the Company or to any of its Subsidiaries through an
instrument duly executed by such Covered Person specifically stating that it is
for use in the preparation thereof. The indemnities of the Company and of its
subsidiaries contained in this Section 1.4.1 shall remain in full force and
effect regardless of any investigation made by or on behalf of such Covered
Person and shall survive any transfer of securities.

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                        1.4.2.              Indemnities to the Company. The
Company and any of its Subsidiaries may require, as a condition to including any
securities in any registration statement filed pursuant to this Agreement, that
the Company and any of its Subsidiaries shall have received an undertaking
satisfactory to it from the prospective seller of such securities, to indemnify
and hold harmless the Company and any of its Subsidiaries, each director of the
Company or any of its Subsidiaries, each officer of the Company or any of its
Subsidiaries who shall sign such registration statement and each other Person
(other than such seller), if any, who controls the Company and any of its
Subsidiaries within the meaning of section 15 of the Securities Act or section
20 of the Exchange Act and each other prospective seller of such securities with
respect to any statement in or omission from such registration statement, any
preliminary prospectus, final prospectus or summary prospectus included therein,
or any amendment or supplement thereto, or any other disclosure document
(including, without limitation, reports and other documents filed under the
Exchange Act or any document incorporated therein) or other document or report,
if such statement or omission was made in reliance upon and in conformity with
written information furnished to the Company or any of its Subsidiaries through
an instrument executed by such seller specifically stating that it is for use in
the preparation thereof. Such indemnity shall remain in full force and effect
regardless of any investigation made by or on behalf of the Company, any of its
Subsidiaries or any such director, officer or controlling Person and shall
survive any transfer of securities.

                        1.4.3.              Contribution. If the indemnification
provided for in Section 1.4.1 or Section 1.4.2 hereof is unavailable to a party
that would have been entitled to indemnification pursuant to the foregoing
provisions of this Section 1.4 (an “Indemnitee”) in respect of any losses,
claims, damages or liabilities (or actions or proceedings in respect
thereof) referred to therein, then each party that would have been an
indemnifying party thereunder shall, in lieu of indemnifying such Indemnitee,
contribute to the amount paid or payable by such Indemnitee as a result of such
losses, claims, damages or liabilities (or actions or proceedings in respect
thereof) in such proportion as is appropriate to reflect the relative fault of
such indemnifying party on the one hand and such Indemnitee on the other in
connection with the statements or omissions which resulted in such losses,
claims, damages or liabilities (or actions or proceedings in respect thereof).
The relative fault shall be determined by reference to, among other things,
whether the untrue or alleged untrue statement of a material fact or the
omission or alleged omission to state a material fact relates to information
supplied by such indemnifying party or such Indemnitee and the parties’ relative
intent, knowledge, access to information and opportunity to correct or prevent
such statement or omission. The parties agree that it would not be just or
equitable if contribution pursuant to this Section 1.4.3 were determined by pro
rata allocation or by any other method of allocation which does not take account
of the equitable considerations referred to in the preceding sentence. The
amount paid or payable by a contributing party as a result of the losses,
claims, damages or liabilities (or actions or proceedings in respect
thereof) referred to above in this Section 1.4.3 shall include any legal or
other expenses reasonably incurred by such Indemnitee in connection with
investigating or defending any such action or claim. No Person guilty of
fraudulent misrepresentation (within the meaning of section 11(f) of the
Securities Act) shall be entitled to contribution from any Person who was not
guilty of such fraudulent misrepresentation.

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                        1.4.4.              Limitation on Liability of Holders
of Registrable Securities. The liability of each holder of Registrable
Securities in respect of any indemnification or contribution obligation of such
holder arising under this Section 1.4 shall not in any event exceed an amount
equal to the net proceeds to such holder (after deduction of all underwriters’
discounts and commissions) from the disposition of the Registrable Securities
disposed of by such holder pursuant to such registration.

                1.5        Reports Under Exchange Act. In order to provide to
the holders of Registrable Securities the benefits of Rule 144 and any other
rule or regulation of the Commission that may at any time permit any such holder
to sell securities of the Company to the public without registration, and in
order to make it possible for the holders of Registrable Securities to register
the sale of the Registrable Securities pursuant to a registration on Form S-3,
the Company agrees to:

    (a)        make and keep public information available, as those terms are
understood and defined in Rule 144;

    (b)        take such action, including the registration of its Common Stock
under section 12 of the Exchange Act, as is necessary to enable the holders of
Registrable Securities to utilize Form S-3 for the sale of their Registrable
Securities (ignoring, for this purpose, the provisions of Items I.A.5 and I.B.3
of the General Instructions thereto);

    (c)        file with the Commission in a timely manner all reports and other
documents required of the Company under the Securities Act and the Exchange Act;
and

    (d)        furnish to any holder of Registrable Securities, so long as the
holder owns any Registrable Securities, forthwith upon request (i) a written
statement by the Company that it has complied with the reporting requirements of
Rule 144, the Securities Act and the Exchange Act, or that it qualifies as a
registrant whose securities may be resold in a secondary offering pursuant to
Form S-3; (ii) a copy of the most recent annual or quarterly report of the
Company filed with the Commission and such other reports and documents so filed
by the Company; and (iii) such other information as may be reasonably requested
in availing any holder of Shares any rule or regulation of the Commission which
permits the selling of any such securities without registration or pursuant to
such form.

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        Section 2.  Definitions. For the purposes of this Agreement, the
following terms have the meanings below:

          “Affiliate” shall mean, with respect to the Company or any of its
Subsidiaries (or any other specified Person), any other Person which, directly
or indirectly controls or is controlled by or is under direct or indirect common
control with the Company or such Subsidiary (or such specified Person), and,
without limiting the generality of the foregoing, shall include (a) any other
Person which beneficially owns or holds 10% of more of any class of voting
securities of such Person or 10% or more of the equity interest in such Person,
(b) any other Person of which such Person beneficially owns or holds 10% or more
of any class of voting securities or in which such Person beneficially owns or
holds 10% or more of the equity interest in such Person and (c) any director or
executive officer of such Person. For the purposes of this definition, the term
“control” (including, with correlative meanings, the terms “controlled by” and
“under common control with”), as used with respect to any Person, means the
possession, directly or indirectly, of the power to direct or cause the
direction of the management and policies of such Person, whether through the
ownership of voting securities or by contract or otherwise.

          “Agreement”shall have the meaning set forth in the first paragraph of
this Agreement.

          “Board”shall mean the Board of Directors of the Company.

          “Commission”shall mean the Securities and Exchange Commission.

          “Common Stock” shall mean the Company’s common stock, par value $0.001
per share.

          “Commonwealth Notes” shall mean an aggregate principal amount of
[$6,100,000] of the Company’s 11.5% Senior Secured Notes due 2005 issued by the
Company to the Holders pursuant to the Securities Exchange Agreement (in
connection with the execution of the Joinder Agreement).

          “Commonwealth Warrants” shall mean the warrants to purchase an
aggregate of [1,116,605] shares of Common Stock of the Company issued by the
Company to the Holders pursuant to the Securities Exchange Agreement (in
connection with the execution of the Joinder Agreement) (including any Warrant
Certificates subsequently issued upon transfer thereof or an exchange therefor
from time to time).

          “Company”has the meaning set forth in the first paragraph of this
Agreement.

          “Covered Person” shall have the meaning set forth in Section 1.4.1.

          “Exchange Act” shall mean the Securities Exchange Act of 1934, as in
effect from time to time.

  -10-

          “Holders” has the meaning set forth in the first paragraph of this
Agreement and, for purposes of this Agreement, shall include any permitted
transferees of such Holders which subsequently acquire Commonwealth Notes or
Commonwealth Warrants.

          “Indemnitee”shall have the meaning set forth in Section 1.4.3.

          “Initiating Holder” shall have the meaning set forth in Section 1.1.1.

          “Joinder Agreement” has the meaning set forth in the second paragraph
of this Agreement.

          “Majority Holders” shall mean, as of any date, collectively, the
holders of a majority of (i) the Commonwealth Warrants (determined on an
as-exercised for cash basis), and (ii) the Commonwealth Notes (determined based
on the aggregate principal amount), in each case outstanding on such date.

          “Majority Participating Holders” shall have the same meaning set forth
in Section 1.1.2.

          “Person” shall mean an individual, a corporation, a partnership, a
limited liability company, a trust, an unincorporated organization or a
government organization or an agency or political subdivision thereof.

          “Public Offering” shall mean a public offering and sale of securities
of the Company for cash pursuant to an effective registration statement under
the Securities Act.

          “Registrable Securities” shall mean the Commonwealth Warrants and the
Commonwealth Notes and all securities directly or indirectly issued or issuable
with respect to the Commonwealth Warrants or the Commonwealth Notes, as the case
may be, by way of a recapitalization, merger, consolidation or other
reorganization; provided, however, that securities which are Registrable
Securities shall cease to be Registrable Securities (i) upon any sale pursuant
to a registration statement or Rule 144 under the Securities Act or (ii) at such
time as they become eligible for sale pursuant to Rule 144(k) under the
Securities Act. Registrable Securities shall also include the Company’s
securities of any other Person that joins this Agreement pursuant to Section 9
below.

          “Rule 144” shall mean Rule 144 under the Securities Act (or any
successor Rule).

          “Rule 145 Transaction” shall mean a registration on Form S-4 pursuant
to Rule 145 of the Securities Act (or any successor Form or provision, as
applicable).

          “Securities Act” shall mean the Securities Act of 1933, as in effect
from time to time.

          “Securities Exchange Agreement” shall have the meaning set forth in
the second paragraph of this Agreement.

  -11-

          “Subsidiary” shall mean, for any Person, (i) a corporation a majority
of whose voting stock is at the time, directly or indirectly, owned by such
Person, by one or more Subsidiaries of such Person or by such Person and one or
more Subsidiaries of such Person, (ii) a partnership in which such Person or a
Subsidiary of such Person is, at the date of determination, a general or limited
partner of such partnership, but only if such Person or its Subsidiary is
entitled to receive more than 50% of the assets of such partnership upon its
dissolution, (iii) a limited liability company, a majority of whose membership
interests is, at the time, directly or indirectly owned by such Person or with
respect to which such Person has a right, under any scenario, to receive 50% or
more of the distributions of the assets of such limited liability company upon
its dissolution, or (iv) any other Person (other than a corporation or
partnership) in which such Person, a Subsidiary of such Person or such Person
and one or more Subsidiaries of such Person, directly or indirectly, at the date
of determination thereof, has (a) at least a majority ownership interest or (b)
the power to elect or direct the election of a majority of the directors or
other governing body of such Person.

          “Warrant Certificates” shall mean certificates evidencing the
Commonwealth Warrants (or evidencing such new warrants issued upon the exchange
or transfer thereof).

        Section 3.  Notices. Any notice or demand authorized by this Agreement
to be given or made by any Holder to or on the Company shall be sufficiently
given or made when and if delivered by facsimile transmission (provided
confirmation of receipt is received immediately thereafter) or when received, if
deposited in the mail, first class or registered, postage prepaid, addressed
(until another address is filed in writing by the Company with the Holders), as
follows:

  iBasis, Inc.
20 Second Avenue
Burlington, MA 01803
Attention: Chief Financial Officer
Facsimile No.: 781-505-7304

with a copy to:

Bingham McCutchen LLP
1900 University Avenue
East Palo Alto, CA 94303-2223
Attn: Johan V. Brigham
Facsimile No.: 650-849-4800

        Any notice pursuant to this Agreement to be given by the Company to any
Holder shall be sufficiently given when and if delivered by facsimile
transmission (provided confirmation of receipt is received immediately
thereafter) or deposited in the mail, first-class or registered, postage
prepaid, addressed (until another address is filed in writing by such Holder
with the Company), as follows:

  -12-

  Commonwealth Advisors, Inc.
P.O. Box 3395
Baton Rouge, LA 70821
Facsimile No.: 225-343-1645

        Section 4.  Successors. All the covenants and provisions of this
Agreement by or for the benefit of the Company shall bind and inure to the
benefit of its successors and assigns hereunder.

        Section 5.  Governing Law; Submission to Jurisdiction: Waiver of Jury
Trial. This Agreement shall be deemed to be a contract made under the laws of
The Commonwealth of Massachusetts and for all purposes shall be governed by and
construed in accordance with the internal laws of said Commonwealth, without
regard to principles of conflicts of laws. Each party hereto hereby submits to
the nonexclusive jurisdiction of the United States District Court for the
District of Massachusetts and of any Massachusetts state court sitting in Boston
for purposes of all legal proceedings arising out of or relating to this
agreement or the transactions contemplated hereby. Each party hereto irrevocably
waives, to the fullest extent permitted by law, any objection which it may now
or hereafter have to the laying of the venue of any such proceeding brought in
such a court and any claim that any such proceeding brought in such a court has
been brought in an inconvenient forum.

        TO THE EXTENT NOT PROHIBITED BY APPLICABLE LAW WHICH CANNOT BE WAIVED,
EACH PARTY HERETO HEREBY 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 OR ACTION, CLAIM, CAUSE OF ACTION OR SUIT (IN CONTRACT,
TORT OR OTHERWISE), INQUIRY, PROCEEDING OR INVESTIGATION ARISING OUT OF OR BASED
UPON THIS AGREEMENT OR THE SUBJECT MATTER HEREOF OR IN ANY WAY CONNECTED WITH OR
RELATED OR INCIDENTAL TO THE TRANSACTIONS CONTEMPLATED HEREBY, IN EACH CASE
WHETHER NOW EXISTING OR HEREAFTER ARISING. EACH PARTY HERETO ACKNOWLEDGES THAT
IT HAS BEEN INFORMED BY THE OTHER PARTIES HERETO THAT THIS SECTION 5 CONSTITUTES
A MATERIAL INDUCEMENT UPON WHICH THEY ARE RELYING AND WILL RELY IN ENTERING INTO
THIS AGREEMENT. Any party hereto may file an original counterpart or a copy of
this Section 5 with any court as written evidence of the consent of each party
hereto to the waiver of its right to trial by jury.

        Section 6.  Exercise of Rights and Remedies. No delay of or omission in
the exercise of any right, power or remedy accruing to any party as a result of
any breach or default by any other party under this Agreement shall impair any
such right, power or remedy, nor shall it be construed as a waiver of or
acquiescence in any such breach or default, or of any similar breach or default
occurring later; nor shall any such delay, omission or waiver of any single
breach or default be deemed a waiver of any other breach or default occurring
before or after that waiver.

        Section 7.  Benefits of This Agreement. Nothing in this Agreement shall
be construed to give to any Person or corporation other than the Company and the
Holders (including any holder of a beneficial interest in any Commonwealth
Warrant or Commonwealth Note) any legal or equitable right, remedy or claim
under this Agreement, and this Agreement shall be for the sole and exclusive
benefit of the Company and the Holders (including any holder of a beneficial
interest in any Commonwealth Warrant or Commonwealth Note).

  -13-

        Section 8.  Counterparts. This Agreement may be executed in any number
of counterparts and each of such counterparts shall for all purposes be deemed
to be an original, and all such counterparts shall together constitute but one
and the same instrument.

        Section 9.  Additional Parties. Any Person that is a holder of the
Company’s 11.5% Senior Secured Notes due 2005 and/or warrants issued in
connection with an exchange of such notes may become a party to this Agreement
by executing and delivering to the Company an Instrument of Accession,
substantially in the form of Annex B attached hereto, provided that the Company
consents to such Person becoming a party to this Agreement (which consent shall
be conclusively deemed to have been given by the Company if and when the Company
countersigns the Instrument of Accession executed by such Person). Any such
Instrument of Accession executed by any such Person and countersigned by the
Company shall become a part of this Agreement. It is hereby understood and
agreed that such Person may become a party to this Agreement without having to
obtain the signature, consent, approval or permission of any of the parties
hereto other than the Company and that, immediately upon such Person becoming a
party to this Agreement, such Person shall be deemed to be a Holder, and Annex A
shall be updated automatically without any action required by the parties
hereto.

[Signature Page Follows]

  -14-

        IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed, as of the day and year first above written.

iBASIS, INC.

By: /s/ Ofer Gneezy
      ——————————————
Name: Ofer Gneezy
Title: President & CEO

HOLDERS listed on Annex A hereto

By: COMMONWEALTH ADVISORS, INC.       their attorney in fact

By: /s/ Walter Morales
      ——————————————
Name: Walter Morales
Title: President