<Page>
                                                                   EXHIBIT 10.70

NEITHER THIS WARRANT NOR THE SHARES ISSUABLE UPON EXERCISE OF THIS WARRANT HAVE
BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR APPLICABLE
STATE SECURITIES LAWS AND NEITHER THIS WARRANT NOR THE SHARES ISSUABLE UPON
EXERCISE OF THIS WARRANT MAY BE SOLD OR TRANSFERRED UNLESS THE REGISTRATION
PROVISIONS OF THE SAID ACT AND APPLICABLE STATE SECURITIES LAWS HAVE BEEN
COMPLIED WITH OR UNLESS COMPLIANCE WITH SUCH PROVISIONS IS NOT REQUIRED. IN
CONNECTION WITH SUCH SALE OR TRANSFER.

                                                              September 24, 2004


                                  iBASIS, INC.

                          COMMON STOCK PURCHASE WARRANT

Void after September 24, 2014

         This Warrant (the "WARRANT") entitles Tejas Securities Group, Inc.
(including any successors or assigns, the "HOLDER"), for value received, to
purchase from iBASIS, INC., a Delaware corporation (the "COMPANY"), at any time
and from time to time, subject to the terms and conditions set forth herein,
during the period starting on the date hereof until 5:00 p.m. on the Expiration
Date (as defined in Section 1 below), at which time this Warrant shall expire
and become void, all or any portion of the Warrant Shares at the Exercise Price
(as defined in Section 1 below). This Warrant is issued subject to the following
terms and conditions:

         1. DEFINITIONS As used in this Warrant, the following terms shall have
the respective meanings set forth below or elsewhere in this Warrant as referred
to below:

         "AFFILIATE" means, with respect to any Person, any other Person
directly or indirectly controlling or controlled by or under direct or indirect
common control with such specified Person. For the purposes of this Agreement,
"control," when used with respect to any specified Person means the power to
direct or cause the direction of the management and policies of such Person,
directly or indirectly, whether through the ownership of voting securities, by
contract or otherwise; and the terms "controlling" and "controlled" have
meanings correlative to the foregoing.

         "BUSINESS DAY" (whether such term is capitalized or not) means any day
except Saturday, Sunday and any day which shall be a federal legal holiday or a
day on which banking institutions in the State of New York are authorized or
required by law or other governmental action to close.

         "COMMON STOCK" means the common stock, $0.001 par value per share, of
the Company (including any securities into which or for which such shares may be
exchanged for, or converted into, pursuant to any stock dividend, stock split,
stock combination, recapitalization, reclassification, reorganization or other
similar event).

         "COMPANY" has the meaning set forth in the preamble hereof.

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         "CURRENT MARKET PRICE" shall mean on any date specified herein, the
average daily Fair Market Value during the period of the most recent 10 days,
ending two trading days immediately preceding such date, on which the national
securities exchanges were open for trading, except that if no Common Stock is
then listed or admitted to trading on any national securities exchange or quoted
in the over-the-counter market, the Current Market Price shall be the Fair
Market Value on such date.

         "EXERCISE PRICE" means, initially, $2.10 per share of Common Stock, as
such amount may be adjusted from time to time pursuant to the terms of this
Warrant.

         "EXPIRATION DATE" means September 24, 2014.

         "FAIR MARKET VALUE" shall mean (i) if the Common Stock is traded on
Nasdaq, then the last reported sale price per share of Common Stock on the
Nasdaq-NMS or any national securities exchange in which such Common Stock is
quoted or listed, as the case may be, on the date immediately preceding each
date the Warrant is exercised or, if no such sale price is reported on such
date, such price on the next preceding business day in which such price was
reported, (ii) if the Common Stock is actively traded over-the-counter, then the
average of the last sales price quoted, if determinable, or, if not
determinable, the closing bid and asked prices quoted on the OTCBB (or similar
system) over the five (5) trading days ended on the trading day immediately
preceding each date the Warrant is exercised or (iii) if such Common Stock is
not traded, quoted or listed on the Nasdaq-NMS or any national securities
exchange or the over-the-counter market, then the fair market value of a share
of Common Stock, as determined in good faith by the Board of Directors of the
Company.

         "HOLDER" has the meaning set forth in the preamble of hereof.

         "PERSON" (whether or not capitalized) means an individual, entity,
partnership, limited liability company, corporation, association, trust, joint
venture, unincorporated organization, and any government, governmental
department or agency or political subdivision thereof.

         "REGISTRATION RIGHTS AGREEMENT" means that certain Registration Rights
Agreement, dated as of the date hereof, as it may be amended from time to time,
by and among the Company and the Investors (as such term is defined therein).

         "SEC" means the United States Securities and Exchange Commission.

         "WARRANT SHARES" means an aggregate of 1,732,500 shares of Common
Stock, subject to adjustment in accordance with Section 3 below.

         2. EXERCISE OF WARRANT.

            2.1 METHOD OF EXERCISE; PAYMENT.

                (a) CASH EXERCISE. Subject to all of the terms and conditions
hereof, this Warrant may be exercised, in whole or in part, at any time and
from time to time during the period commencing on the date hereof and ending
on the Expiration Date, by surrender of this Warrant to the Company at its
principal office, accompanied by a subscription substantially in the form
attached hereto, executed by the Holder and accompanied by (a) wire transfer
of immediately available funds or (b) certified or official bank check
payable to the order of the Company, in each case in the amount obtained by
multiplying (i) the number of Warrant Shares (without giving effect to any
adjustment thereof pursuant to the provisions of hereof) for which the
Warrant is being exercised, as designated in such subscription, by (ii) the
Exercise Price. Thereupon, the Holder shall be entitled to receive the number
of duly

                                      -2-
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authorized, validly issued, fully paid and nonassessable Warrant Shares
determined as provided for herein.

                (b) CONVERSION. Subject to all of the terms and conditions
hereof, the Holder shall have the right to convert this Warrant, in whole or
in part, at any time and from time to time during the period commencing on
the date hereof and ending on the Expiration Date, by surrender of this
Warrant to the Company at its principal office, accompanied by a conversion
notice substantially in the form attached hereto, executed by the Holder.
Thereupon, the Holder shall be entitled to receive a number of duly
authorized, validly issued, fully paid and nonassessable Warrant Shares equal
to:

                    (i)  the excess of

                         (A) (x) the number of Warrant Shares determined as
                    provided in Section 3 hereof which such holder would be
                    entitled to receive upon exercise of such Warrant for the
                    number of Warrant Shares designated in such conversion
                    notice (without giving effect to any adjustment thereof
                    pursuant to the provisions of this Section 2.1(b)) for
                    which the Warrant is then being exercised, as designated
                    in such conversion notice, MULTIPLIED BY (y) the Current
                    Market Price of each such Warrant Share so receivable upon
                    such exercise

                    OVER

                         (B) (x) the number of Warrant Shares (determined as
                    provided in Section 3) which such holder would be entitled
                    to receive upon exercise of such Warrant for the number of
                    Warrant Shares designated in such conversion notice (without
                    giving effect to any adjustment thereof pursuant to the
                    provisions of this Section 2.1(b)), MULTIPLIED BY (y) the
                    Exercise Price

                    DIVIDED BY

                    (ii) such Current Market Price of each such Warrant Share.

            2.2 WHEN EXERCISE EFFECTIVE. Each exercise of this Warrant shall
be deemed to have been effected immediately prior to the close of business on
the business day on which this Warrant shall have been surrendered to the
Company as provided in Section 2.1, and at such time the Person or Persons in
whose name or names any certificate or certificates for Warrant Shares shall
be issuable upon such exercise as provided in Section 2.3 shall be deemed to
have become the holder or holders of record thereof.

            2.3 DELIVERY OF STOCK CERTIFICATES ON EXERCISE. As soon as
practicable after the exercise of this Warrant, and in any event within three
(3) business days thereafter, the Company, at its expense, and in accordance
with applicable securities laws, will cause to be issued in the name of and
delivered to the Holder, or as the Holder may direct (subject in all cases,
to the provisions of Section 8 hereof), a certificate or certificates for the
number of Warrant Shares purchased by the Holder on such exercise, PLUS, in
lieu of any fractional share to which the Holder would otherwise be entitled,
cash equal to such fraction multiplied by the Fair Market Value.

                                      -3-
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            2.4 SHARES TO BE FULLY PAID AND NONASSESSABLE. All Warrant Shares
issued upon the exercise of this Warrant shall be validly issued, fully paid
and nonassessable, free of all liens, taxes, charges and other encumbrances
or restrictions on sale (other than those set forth herein).

            2.5 ISSUANCE OF NEW WARRANTS; COMPANY ACKNOWLEDGMENT. Upon any
partial exercise of this Warrant, the Company, at its expense, will forthwith
and, in any event within three (3) business days, issue and deliver to the
Holder a new warrant or warrants of like tenor, registered in the name of the
Holder, exercisable, in the aggregate, for the balance of the Warrant Shares.
Moreover, the Company shall, at the time of any exercise of this Warrant,
upon the request of the Holder, acknowledge in writing its continuing
obligation to afford to the Holder any rights to which the Holder shall
continue to be entitled after such exercise in accordance with the provisions
of this Warrant; PROVIDED, HOWEVER, that if the Holder shall fail to make any
such request, such failure shall not affect the continuing obligation of the
Company to afford to the Holder any such rights.

            2.6 PAYMENT OF TAXES AND EXPENSES. The Company shall pay any
recording, filing, stamp or similar tax which may be payable in respect of
any transfer involved in the issuance of, and the preparation and delivery of
certificates (if applicable) representing, (i) any Warrant Shares purchased
upon exercise of this Warrant and/or (ii) new or replacement warrants in the
Holder's name or the name of any transferee of all or any portion of this
Warrant.

         3. ADJUSTMENT OF EXERCISE PRICE. The Exercise Price shall be subject
to adjustment from time to time upon the happening of certain events as
follows:

            3.1 SUBDIVISION OR COMBINATION OF STOCK. If at any time or from
time to time after the date hereof, the Company shall subdivide (by way of
stock dividend, stock split or otherwise) its outstanding shares of Common
Stock, the Exercise Price in effect immediately prior to such subdivision
shall be reduced proportionately and the number of Warrant Shares (calculated
to the nearest whole share) shall be increased proportionately, and
conversely, in the event the outstanding shares of Common Stock shall be
combined (whether by stock combination, reverse stock split or otherwise)
into a smaller number of shares, the Exercise Price in effect immediately
prior to such combination shall be increased proportionately and the number
of Warrant Shares (calculated to the nearest whole share) shall be decreased
proportionately. The Exercise Price and the number of Warrant Shares, as so
adjusted, shall be readjusted in the same manner upon the happening of any
successive event or events described in this Section 3.1.

            3.2 ADJUSTMENT FOR STOCK DIVIDENDS. If at any time after the date
hereof, the Company shall declare a dividend or make any other distribution
upon any class or series of stock of the Company payable in shares of Common
Stock or securities convertible into shares of Common Stock, the Exercise
Price and the number of Warrant Shares to be obtained upon exercise of this
Warrant shall be adjusted proportionately to reflect the issuance of any
shares of Common Stock or convertible securities, as the case may be,
issuable in payment of such dividend or distribution. The Exercise Price and
the number of Warrant Shares, as so adjusted, shall be readjusted in the same
manner upon the happening of any successive event or events described in this
Section 3.2.

            3.3 ADJUSTMENTS FOR RECLASSIFICATIONS. If the Common Stock
issuable upon the conversion of this Warrant shall be changed into the same
or a different number of shares of any class(es) or series of stock, whether
by reclassification or otherwise (other than an adjustment under Sections 3.1
and 3.2 or a merger, consolidation, or sale of assets provided for under
Section 3.4), then and in each such event, the Holder hereof shall have the
right thereafter to convert each Warrant Share into the kind and amount of
shares of stock and other securities and property receivable upon such
reclassification, or other change by holders of the number of shares of
Common Stock into which such Warrant Shares would have

                                      -4-
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been convertible immediately prior to such reclassification or change, all
subject to successive adjustments thereafter from time to time pursuant to and
in accordance with, the provisions of this Section 3.

            3.4 ADJUSTMENTS FOR MERGER OR CONSOLIDATION. In the event that,
at any time or from time to time after the date hereof, the Company shall (a)
effect a reorganization, (b) consolidate with or merge into any other Person,
or (c) sell or transfer all or substantially all of its properties or assets
or more than 50% of the voting capital stock of the Company (whether issued
and outstanding, newly issued, from treasury, or any combination thereof) to
any other person under any plan or arrangement contemplating the
consolidation or merger, sale or transfer, or dissolution of the Company,
then, in each such case, the Holder, upon the exercise of this Warrant as
provided in Section 2.1 hereof at any time or from time to time after the
consummation of such reorganization, consolidation, merger or sale or the
effective date of such dissolution, as the case may be, shall receive, in
lieu of the Warrant Shares issuable on such exercise immediately prior to
such consummation or such effective date, as the case may be, the stock and
property (including cash) to which the Holder would have been entitled upon
the consummation of such consolidation or merger, or sale or transfer, or in
connection with such dissolution, as the case may be, if the Holder had so
exercised this Warrant immediately prior thereto (assuming the payment by the
Holder of the Exercise Price therefor as required hereby in a form permitted
hereby, which payment shall be included in the assets of the Company for the
purposes of determining the amount available for distribution), all subject
to successive adjustments thereafter from time to time pursuant to, and in
accordance with, the provisions of this Section 3.

            3.5 CONTINUATION OF TERMS. Upon any reorganization,
consolidation, merger or transfer (and any dissolution following any such
transfer) referred to in this Section 3, this Warrant shall continue in full
force and effect and the terms hereof shall be applicable to the shares of
Common Stock and other securities and property receivable upon the exercise
of this Warrant after the consummation of such reorganization, consolidation
or merger or the effective date of dissolution following any such transfer,
as the case may be, and shall be binding upon the issuer of any such Common
Stock or other securities, including, in the case of any such transfer, the
Person acquiring all or substantially all of the properties or assets or more
than 50% of the voting capital stock of the Company (whether issued and
outstanding, newly issued or from treasury or any combination thereof),
whether or not such Person shall have expressly assumed the terms of this
Warrant.

            3.6 CERTIFICATE AS TO ADJUSTMENTS. Upon the occurrence of each
adjustment or readjustment of the Exercise Price and number of Warrant Shares
pursuant to this Section 3, this Warrant shall, without any action on the
part of the Holder, be adjusted in accordance with this Section 3, and the
Company, at its expense, promptly shall compute such adjustment or
readjustment in accordance with the terms hereof and prepare and furnish to
the Holder a certificate setting forth such adjustment or readjustment,
showing in detail the facts upon which such adjustment or readjustment is
based. The Company will forthwith send a copy of each such certificate to the
Holder in accordance with Section 10.4 below.

         4. REGISTRATION RIGHTS. The initial holders of the Warrant Shares
and any permitted transferees of such rights in accordance with the terms
thereof shall be entitled to the registration rights and other rights
applicable to such shares provided by the Registration Rights Agreement.

         5. NOTICES OF RECORD DATE. Upon (a) any establishment by the Company of
a record date of the holders of any class of securities for the purpose of
determining the holders thereof who are entitled to receive any dividend or
other distribution, or right or option to acquire securities of the Company, or
any other right, or (b) any capital reorganization, reclassification,
recapitalization, merger or consolidation of the Company with or into any other
corporation, any transfer of all or substantially all the assets of the

                                      -5-
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Company, or any voluntary or involuntary dissolution, liquidation or winding up
of the Company, or the sale, in a single transaction, of a majority of the
Company's voting stock (whether newly issued, or from treasury, or previously
issued and then outstanding, or any combination thereof), the Company shall mail
to the Holder at least ten (10) business days, or such longer period as may be
required by law, prior to the record date specified therein, a notice specifying
(i) the date established as the record date for the purpose of such dividend,
distribution, option or right and a description of such dividend, distribution,
option or right, (ii) the date on which any such reorganization,
reclassification, transfer, consolidation, merger, dissolution, liquidation or
winding up, or sale is expected to become effective and (iii) the date, if any,
fixed as to when the holders of record of Common Stock shall be entitled to
exchange their shares of Common Stock for securities or other property
deliverable upon such reorganization, reclassification, transfer, consolidation,
merger, dissolution, liquidation or winding up.

         6. EXCHANGE OF WARRANT. Subject to the provisions of Section 8 hereof
(if and to the extent applicable), this Warrant shall be exchangeable, upon the
surrender hereof by the Holder at the principal office of the Company, for new
warrants of like tenor, each registered in the name of the Holder or, subject to
compliance with applicable federal and state securities laws, in the name of
such other persons as the Holder may direct (upon payment by the Holder of any
applicable transfer taxes). Each of such new warrants shall be exercisable for
such number of Warrant Shares as the Holder shall direct, PROVIDED that all of
such new warrants shall represent, in the aggregate, the right to purchase the
same number of Warrant Shares and cash, securities or other property, if any,
which may be purchased by the Holder upon exercise of this Warrant at the time
of its surrender.

         7. NO DILUTION OR IMPAIRMENT. The Company will not, by amendment of its
Certificate of Incorporation or By-Laws or through any reorganization, transfer
of assets, consolidation, merger, dissolution, issue or sale of securities or
any other voluntary action, avoid or seek to avoid the observance or performance
of any of the terms of this Warrant, but will at all times in good faith assist
in the carrying out of all such terms and in the taking of all action as may be
necessary or appropriate in order to protect the rights of the Holder against
dilution, or other impairment.

         8. TRANSFER PROVISIONS, ETC.

            8.1 LEGENDS. Each certificate representing any Warrant Shares
issued upon exercise of this Warrant shall bear the following legend:

         "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE NOT BEEN REGISTERED
         UNDER THE SECURITIES ACT OF 1933, AS AMENDED, AND MAY NOT BE OFFERED OR
         SOLD IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT UNDER SAID
         ACT OR PURSUANT TO AN AVAILABLE EXEMPTION FROM, OR IN A TRANSACTION NOT
         SUBJECT TO, REGISTRATION UNDER SAID ACT."

            8.2 MECHANICS OF TRANSFER.

                (a) Any transfer of all or any portion of this Warrant (and
the Warrant Shares), or of any interest herein or therein, that is otherwise
in compliance with applicable law shall be effected by surrendering this
Warrant to the Company at its principal office, together with a duly executed
form of assignment, in the form attached hereto. In the event of any such
transfer of this Warrant, subject to compliance with applicable federal and
state securities laws, the Company shall issue a new warrant or warrants of
like tenor to the transferee(s), representing, in the aggregate, the right to
purchase the same number of Warrant Shares and cash, securities or other
property, if any, which may be purchased by the Holder upon exercise of this
Warrant at the time of its surrender.

                                      -6-
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                            (b) In the event of any transfer of all or any
portion of this Warrant in accordance with Section 8.2(a) above, the Company
shall issue (i) a new warrant of like tenor to the transferee, representing the
right to purchase the number of Warrant Shares, and cash, securities or other
property, if any, which were purchasable by the Holder of the transferred
portion of this Warrant, and (ii) a new warrant of like tenor to the Holder,
representing the right to purchase the number of Warrant Shares, and cash,
securities or other property, if any, purchasable by the Holder of the
un-transferred portion of this Warrant. Until this Warrant or any portion
thereof is transferred on the books of the Company, the Company may treat the
Holder as the absolute holder of this Warrant and all right, title and interest
therein for all purposes, notwithstanding any notice to the contrary.

                  8.3 NO RESTRICTIONS ON TRANSFER. Subject to compliance with
applicable federal and state securities laws, this Warrant and any portion
hereof, the Warrant Shares and the rights hereunder may be transferred by the
Holder in its sole discretion at any time and to any Person or Persons,
including without limitation Affiliates and affiliated groups of such Holder,
without the consent of the Company.

                  8.4 RECORDS. The Company shall keep a record of the name and
address of each holder of the Warrant, each transfer of the Warrant, and the
name and address of each transferee of the Warrant.

         9. LOST, STOLEN OR DESTROYED WARRANT. Upon receipt by the Company of
evidence satisfactory to it of loss, theft, destruction or mutilation of this
Warrant and, in the case of loss, theft or destruction, on delivery of a
customary affidavit of the Holder and indemnity agreement, or, in the case of
mutilation, upon surrender of this Warrant, the Company at its expense will
execute and deliver, or will instruct its transfer agent to execute and deliver,
a new Warrant of like tenor and date, and any such lost, stolen or destroyed
Warrant thereupon shall become void.

        10. GENERAL.

            10.1 AUTHORIZED SHARES, RESERVATION OF SHARES FOR ISSUANCE. At
all times while this Warrant is outstanding, the Company shall maintain its
corporate authority to issue, and shall have authorized and reserved for
issuance upon exercise of this Warrant, such number of shares of Common
Stock, any other capital stock or other securities as shall be sufficient to
perform its obligations under this Warrant (after giving effect to any and
all adjustments to the number and kind of Warrant Shares purchasable upon
exercise of this Warrant).

            10.2 NO IMPAIRMENT. The Company will not, by amendment of its
Certificate of Incorporation or through any reorganization, transfer of
assets, consolidation, merger, dissolution, issuance or sale of securities,
sale or other transfer of any of its assets or properties, or any other
voluntary action, avoid or seek to avoid the observance or performance of any
of the terms of this Warrant, but will at all times in good faith assist in
the carrying out of all such terms and in the taking of all such action as
may be necessary or appropriate in order to protect the rights of the Holder
hereunder against impairment. Without limiting the generality of the
foregoing, the Company (a) will not increase the par value of any shares of
Common Stock receivable upon the exercise of this Warrant above the amount
payable therefor on such exercise, and (b) will take all action that may be
necessary or appropriate in order that the Company may validly and legally
issue fully paid and nonassessable shares of Common Stock upon the exercise
of this Warrant.

            10.3 NO RIGHTS AS STOCKHOLDER. The Holder shall not be entitled
to vote or to receive dividends or to be deemed the holder of Common Stock
that may at any time be issuable upon exercise of this Warrant for any
purpose whatsoever, nor shall anything contained herein be construed to
confer upon the Holder any of the rights of a stockholder of the Company or
any right to vote for the election of

                                      -7-
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directors or upon any matter submitted to stockholders at any meeting thereof,
or to give or withhold consent to any corporate action (whether upon any
recapitalization, issuance or reclassification of stock, change of par value or
change of stock to no par value, consolidation, merger or conveyance or
otherwise), or to receive notice of meetings (except to the extent otherwise
provided in this Warrant), or to receive dividends or subscription rights, until
the Holder shall have exercised this Warrant and been issued Warrant Shares in
accordance with the provisions hereof.

            10.4 NOTICES. All notices, requests, consents and other
communications hereunder shall be in writing and shall be deemed to have been
given if personally delivered or delivered by overnight courier or mailed by
first-class registered or certified mail, postage prepaid, return receipt
requested, or sent by fax machine, addressed as follows:

                           (a)      if to the Company at:

                                    iBasis, Inc.
                                    20 Second Avenue
                                    Burlington, MA  01803
                                    Attention:  Chief Financial Officer

                                    with a copy to:
                                    Johan V. Brigham
                                    Bingham McCutchen LLP
                                    150 Federal Street
                                    Boston, MA 02110

                           (b)      if to the Holder, at the Holder's address
appearing in the books maintained by the Company.

        11. AMENDMENT AND WAIVER. No failure or delay of the Holder in
exercising any power or right hereunder shall operate as a waiver thereof,
nor shall any single or partial exercise of any such right or power, or any
abandonment or discontinuance of steps to enforce such a right or power,
preclude any other or further exercise thereof or the exercise of any other
right or power. The rights and remedies of the Holder are cumulative and not
exclusive of any rights or remedies which it would otherwise have. The terms
of this Warrant may be amended, modified or waived only with the written
consent of the Company and the Holder.

        12. GOVERNING LAW. This Warrant shall be governed by and construed in
accordance with the laws of the State of New York, as such laws are applied
to contracts entered into and wholly to be performed within the State of New
York and without giving effect to any principles of conflicts or choice of
law that would result in the application of the laws of any other
jurisdiction.

        13. COVENANTS TO BIND SUCCESSOR AND ASSIGNS. All covenants,
stipulations, promises and agreements in this Warrant contained by or on
behalf of the Company shall bind its successors and assigns, whether so
expressed or not.

        14. SEVERABILITY. In case any one or more of the provisions contained
in this Warrant shall be invalid, illegal or unenforceable in any respect,
the validity, legality and enforceability of the remaining provisions
contained herein shall not in any way be affected or impaired thereby. The
parties shall endeavor in good faith negotiations to replace the invalid,
illegal or unenforceable provisions with valid provisions the economic effect
of which comes as close as possible to that of the invalid, illegal or
unenforceable provisions.

                                      -8-
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        15. CONSTRUCTION. The definitions of this Warrant shall apply equally
to both the singular and the plural forms of the terms defined. Wherever the
context may require, any pronoun shall include the corresponding masculine,
feminine and neuter forms. The section and paragraph headings used herein are
for convenience of reference only, are not part of this Warrant and are not
to affect the construction of or be taken into consideration in interpreting
this Warrant.

        16. REMEDIES. The Holder, in addition to being entitled to exercise
all rights granted by law, including recovery of damages, will be entitled to
specific performance of its rights under this Warrant. The Company agrees
that monetary damages would not be adequate compensation for any loss
incurred by reason of a breach by it of the provisions of this Warrant and
hereby agrees to waive the defense in any action for specific performance
that a remedy at law would be adequate. In any action or proceeding brought
to enforce any provision of this Warrant or where any provision hereof is
validly asserted as a defense, the successful party to such action or
proceeding shall be entitled to recover reasonable attorneys' fees in
addition to any other available remedy.

                           [SIGNATURE PAGE TO FOLLOW]

                                      -9-
<Page>

     IN WITNESS WHEREOF, the Company has executed this Common Stock Purchase
Warrant as of the date first written above.


                                         COMPANY:

                                         iBASIS, INC.


                                         By: /s/ OFER GNEEZY
                                             ----------------------------------
                                         Name:    Ofer Gneezy
                                         Title:   President and Chief Executive
                                                  Officer

<Page>

                                   NOTICE AND
                                  SUBSCRIPTION


To:      iBasis, Inc.
         20 Second Avenue
         Burlington, MA  01803


         The undersigned hereby irrevocably elects to exercise the right of
purchase represented by the attached Warrant for, and to exercise thereunder,
__________ shares of Common Stock, of iBASIS, INC., a Delaware corporation (the
"COMPANY"), and tenders herewith payment of $__________, representing the
aggregate purchase price for such shares based on the price per share provided
for in such Warrant. Such payment is being made in accordance with Section
2.1(a) of the attached Warrant.

         The undersigned hereby represents and warrants as follows:

         (a) the undersigned is acquiring such shares of Common Stock for its
own account for investment and not for resale or with a view to distribution
thereof in violation of the Securities Act; and

         (b) the undersigned is an "accredited investor" as defined in Rule 501
of Regulation D promulgated under the Securities Act and was not organized for
the purpose of acquiring the Warrant or such shares of Common Stock. The
undersigned's financial condition is such that it is able to bear the risk of
holding such securities for an indefinite period of time and the risk of loss of
its entire investment. The undersigned has sufficient knowledge and experience
in investing in companies similar to the Company so as to be able to evaluate
the risks and merits of its investment in the Company.

         Please issue a certificate or certificates for such shares of Common
Stock in the following name or names and denominations and deliver such
certificate or certificates to the person or persons listed below at their
respective addresses set forth below:

                             ---------------------
                             ---------------------
                             ---------------------
                             ---------------------

         If said number of shares of Common Stock shall not be all the shares of
Common Stock issuable upon exercise of the attached Warrant, a new Warrant is to
be issued in the name of the undersigned for the balance remaining of such
shares of Common Stock less any fraction of a share of Common Stock paid in
cash.

Dated:  ___________, ____                    _________________________________
                                             Signature

<Page>

         The undersigned iBasis, Inc. hereby acknowledges receipt of this Notice
and Subscription and authorizes issuance of the shares of Common Stock described
above.

iBASIS, INC.

By:
Title:
Date:

<Page>

                               FORM OF ASSIGNMENT

                   (To be executed upon assignment of Warrant)

         For value received, __________________________________ hereby sells,
assigns and transfers unto __________________ the attached Warrant [__% of the
attached Warrant], together with all right, title and interest therein, and does
hereby irrevocably constitute and appoint ____________________ attorney to
transfer said Warrant [said percentage of said Warrant] on the books of iBASIS,
INC., a Delaware corporation, with full power of substitution in the premises.

         If not all of the attached Warrant is to be so transferred, a new
Warrant is to be issued in the name of the undersigned for the balance of said
Warrant.

         The undersigned hereby agrees that it will not sell, assign, or
transfer the right, title and interest in and to the Warrant unless applicable
federal and state securities laws have been complied with.



Dated:  _____________, ____         ________________________________________
                                    Signature

<Page>

                            FORM OF CONVERSION NOTICE

To iBasis, Inc.:

         The undersigned registered holder of the attached Warrant hereby
irrevocably converts such Warrants with respect to ________(1) shares of the
Common Stock which such holder would be entitled to receive upon the exercise
hereof, and requests that the certificates for such shares be issued in the name
of, and delivered to _________________, whose address is as follows:

                             ---------------------
                             ---------------------
                             ---------------------
                             ---------------------

         Such conversion is being made in accordance with Section 2.1(b) of the
attached Warrant. The undersigned hereby represents and warrants as follows:

         (a) the undersigned is acquiring such shares of Common Stock for its
own account for investment and not for resale or with a view to distribution
thereof in violation of the Securities Act; and

         (b) the undersigned is an "accredited investor" as defined in Rule 501
of Regulation D promulgated under the Securities Act and was not organized for
the purpose of acquiring the Warrant or such shares of Common Stock. The
undersigned's financial condition is such that it is able to bear the risk of
holding such securities for an indefinite period of time and the risk of loss of
its entire investment. The undersigned has sufficient knowledge and experience
in investing in companies similar to the Company so as to be able to evaluate
the risks and merits of its investment in the Company.

Dated:
                      _________________________________________________________
                      (Signature must conform in all respects to name of holder
                      as specified on the face of Warrant)

                      _________________________________________________________
                      (Street Address)

                      _________________________________________________________
                      (City)            (State)          (Zip Code)


------------------
(1) Insert here the number of shares called for on the face of this Warrant
Certificate (or, in the case of a partial conversion, the portion thereof as to
which the Warrants evidenced by this Warrant Certificate are being converted),
in either case without making any adjustment for additional shares of Common
Stock or any other stock or other securities or property or cash which, pursuant
to the adjustment provisions of the Warrant evidenced by this Warrant
Certificate, may be delivered upon exercise. In the case of a partial
conversion, a new Warrant Certificate will be issued and delivered, representing
the unconverted portion of the Warrants, to the holder surrendering this Warrant
Certificate.

<Page>

         The undersigned iBasis, Inc. hereby acknowledges receipt of this
Conversion Notice and authorizes issuance of the shares of Common Stock
described above.

iBASIS, INC.

By: _______________________________
Title: ____________________________
Date: _____________________________