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                                                                   Exhibit 10.63

                                                                  EXECUTION COPY

                               SECURITY AGREEMENT

          This Security Agreement (this "Agreement") is made as of the 18TH DAY
OF JUNE, 2004, by and between IBASIS HOLDINGS, INC., a Delaware corporation with
its principal place of business at 20 Second Avenue, Burlington, Massachusetts
01803 ("Grantor"), and THE BANK OF NEW YORK, a New York banking corporation, as
Collateral Agent for the Holders (as defined in the Indenture referred to
below), with its principal place of business at 101 Barclay Street, Floor 810,
New York, New York 10286 ("Secured Party").

                                    RECITALS

          A.     Pursuant to that certain Indenture (as the same may be amended
or supplemented from time to time, the "Indenture"), dated June 18, 2004, by and
among IBASIS, Inc. (the "Company"), the Grantor, the other subsidiary guarantors
named therein and The Bank of New York, as Trustee (the "Trustee), the Company
has issued its 8% Senior Subordinated Convertible Notes due 2007 ("Notes") in an
aggregate principal amount not to exceed $29,000,000. The Notes are secured in
part pursuant to the terms of the Security Documents (as defined in the
Indenture), and this Agreement is one of the Security Documents. Grantor is a
Wholly Owned Subsidiary (as defined in the Indenture) of IBASIS, Inc. and will
receive a substantial and material benefit from the transactions contemplated by
the Indenture and this Agreement.

          B.     In accordance with the terms of the Indenture, Grantor desires
to grant Secured Party a Lien (as defined in the Indenture) upon the Collateral
(hereinafter defined).

          C.     Unless otherwise indicated herein, capitalized terms not
defined herein shall have the same meaning as those provided in the Indenture.

                                    AGREEMENT

          The parties agree as follows:

     1.   CREATION OF SECURITY INTEREST

          1.1    GRANT OF SECURITY INTEREST. Grantor grants to Secured Party,
for the benefit of the Holders, the Trustee and the Secured Party, a security
interest in the property described in EXHIBIT A attached hereto (the
"Collateral") to secure any and all liabilities and obligations to the Holders,
the Trustee and the Secured Party under the Notes and the Indenture (the
"Indebtedness"). Such security interest constitutes at least a priority security
interest, subject only to the Senior Indebtedness, in the presently existing
Collateral, and in Collateral acquired after the date hereof.

          1.2    FINANCING STATEMENTS. This Agreement constitutes an
authenticated record which authorizes the Secured Party to file such financing
statements as Secured Party reasonably determines appropriate. Without limiting
the generality of the foregoing, Grantor hereby expressly authorizes Secured
Party to file financing statements without notice to Grantor, with all
appropriate jurisdictions, as Secured Party in its reasonable discretion deems
appropriate from

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time to time, in order to further perfect, protect, or vest more securely
Secured Party's interest in the Collateral.

          1.3    DELIVERY OF ADDITIONAL DOCUMENTATION REQUIRED. Grantor shall
from time to time execute and deliver to Secured Party, at the request of
Secured Party, all financing statements and other documents that Secured Party
may reasonably request, in form satisfactory to Secured Party, to perfect and
continue perfected Secured Party's security interests in the Collateral and in
order to fully consummate all of the transactions contemplated under the
Indenture.

     2.   REPRESENTATIONS AND WARRANTIES

          Grantor represents and warrants as follows:

          2.1    DUE ORGANIZATION AND QUALIFICATION. Grantor is a corporation
duly existing and in good standing under the laws of its state of incorporation
and qualified and licensed to do business in, and is in good standing in, any
state in which the conduct of its business or its ownership of property requires
that it be so qualified. Grantor represents and warrants that its legal name,
type of organization, jurisdiction or incorporation, organizational
identification number and place of business are indicated on Schedule 1 attached
hereto. Grantor shall not, without at least thirty (30) days prior written
notice to Secured Party: (a) relocate its chief executive office, or add any new
offices or business locations, or (b) change its jurisdiction of organization,
or (c) change its organizational structure or type, or (d) change its legal
name, or (e) change any organizational number (if any) assigned by its
jurisdiction of organization.

          2.2    DUE AUTHORIZATION; NO CONFLICT. The execution, delivery, and
performance of this Agreement are within Grantor's powers, have been duly
authorized, and are not in conflict with nor constitute a breach of any
provision contained in Grantor's organizational documents, nor will they
constitute an event of default under any material agreement to which Grantor is
a party or by which Grantor is bound.

          2.3    NO PRIOR ENCUMBRANCES. Grantor has good and indefeasible title
to the Collateral, free and clear of any liens, security interests, or other
encumbrances other than the Liens permitted by the Indenture.

          2.4    NO INTELLECTUAL PROPERTY. Grantor holds no interest in any
trade secret, copyright, patent or other intellectual property.

     3.   AFFIRMATIVE COVENANTS

          Grantor covenants and agrees that, until payment in full of all
outstanding obligations and liabilities under the Indebtedness and this
Agreement, Grantor shall do all of the following:

          3.1    GOOD STANDING. Grantor shall maintain its corporate existence
and its good standing in its jurisdiction of incorporation and maintain
qualification in each jurisdiction in which the failure to so qualify could have
a material adverse effect on Grantor's business.

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Grantor shall maintain in force all licenses, approvals and agreements, the loss
of which could have a material adverse effect on Grantor's business.

          3.2    GOVERNMENT COMPLIANCE. Grantor shall comply with all statutes,
laws, ordinances and government rules and regulations to which it is subject,
noncompliance with which could have a material adverse effect on Grantor's
business.

          3.3    INSURANCE.

                 (a)  Grantor, at its expense, shall keep the Collateral insured
against loss or damage by fire, theft, explosion, sprinklers, and all other
hazards and risks, and in such amounts, as ordinarily insured against by other
owners in similar businesses conducted in the locations where Grantor's business
is conducted on the date hereof. Grantor shall also maintain insurance relating
to Grantor's ownership and use of the Collateral in amounts and of a type that
are customary to businesses similar to Grantor's.

                 (b)  All such policies of insurance shall be in such form, with
such companies, and in such amounts as reasonably satisfactory to Secured Party.
From and after such time as the Senior Indebtedness has been indefeasibly
repaid, in full, in cash, and all commitments thereunder have been terminated,
all such policies of property insurance shall contain a lender's loss payable
endorsement, in a form satisfactory to Secured Party, showing Secured Party as
an additional loss payee thereof and all liability insurance policies shall show
the Secured Party as an additional insured, and shall specify that the insurer
must give at least twenty (20) days notice to Secured Party before canceling its
policy for any reason.

          3.4    INTELLECTUAL PROPERTY. So long as any liability under the Notes
or the Indenture remains outstanding, Grantor shall notify the Secured Party if
Grantor should acquire any interest in any trade secret, copyright, patent or
other intellectual property, which notice shall be sent within forty-five (45)
days of Grantor's acquisition of such interest.

     4.   NEGATIVE COVENANTS

          Except as permitted by the Indenture, Grantor covenants and agrees
that until payment in full of all outstanding obligations and liabilities under
the Indebtedness and this Agreement, Grantor will not do any of the following:

          4.1    DISPOSITIONS. Convey, sell, lease, transfer or otherwise
dispose of (collectively, a "Transfer"), all or any part of the Collateral other
than (a) Transfers in the ordinary course of business; (b) Transfers of
non-exclusive licenses and similar arrangements for the use of the Collateral;
or (c) Transfers of worn-out or obsolete Collateral.

          4.2    MERGERS OR ACQUISITIONS. Merge or consolidate, or permit any of
its subsidiaries to merge or consolidate, with any other Person, or acquire, or
permit any of its Subsidiaries to acquire, all or substantially all of the
capital stock or property of another Person.

          4.3    INDEBTEDNESS. Create, incur, assume, or be liable for any
Indebtedness, or permit any Subsidiary to do so, other than indebtedness subject
to a subordination agreement between the Secured Party and the holder of such
indebtedness ("Subordinated Debt").

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          4.4    ENCUMBRANCE. Create, incur, or allow any Lien on any of its
property, or assign or convey any right to receive income, including the sale of
any receivables, or permit any of its Subsidiaries to do so, or permit any
Collateral not to be subject to Secured Party's security interest.

          4.5    INVESTMENTS; DISTRIBUTIONS. Directly or indirectly acquire or
own any Person, or make any Investments in any Person, other than investments
permitted in writing by Secured Party, or permit any of its Subsidiaries to do
so; or (b) pay any dividends or make any distribution or payment or redeem,
retire or purchase any capital stock (except for dividends payable solely in
stock of Grantor).

          4.6    TRANSACTIONS WITH AFFILIATES. Directly or indirectly enter or
permit any material transaction with any Affiliate, except transactions that are
in the ordinary course of Grantor's business, on terms no less favorable to
Grantor than would be obtained in an arm's length transaction with a
non-affiliated Person.

          4.7    SUBORDINATED DEBT. Make or permit any payment on any
subordinated debt, except under the terms of the Subordinated Indebtedness, or
amend any material provision in any document relating to the Subordinated
Indebtedness, without Secured Party's prior written consent.

          4.8    COMPLIANCE. Undertake as one of its important activities
extending credit to purchase or carry margin stock, or use the proceeds of any
advance for that purpose; fail to comply with the Federal Fair Labor Standards
Act or violate any other law or regulation, if the violation could reasonably be
expected to have a material adverse effect on Grantor's business or operations,
or permit any of its Subsidiaries to do so.

     5.   EVENTS OF DEFAULT

          Any one or more of the following events shall constitute an Event of
Default by Grantor under this Agreement:

          5.1    INDENTURE. If an Event of Default occurs under the Indenture.

          5.2    COVENANT DEFAULT. Grantor does not perform any obligation in
Article 3 or violates any covenant in Article 4 or does not perform or observe
any other material term, condition or covenant in this Agreement, and the
continuance of such failure for a period of 45 days after the date on which
written notice of such failure, requiring the Grantor to remedy the same, shall
have been given to Grantor and a Responsible Officer of the Trustee by the
holders of at least 25% of aggregate principal amount of the outstanding Notes
at the time outstanding determined in accordance with Section 9.4 of the
Indenture.

          5.3    MATERIAL ADVERSE CHANGE. A material impairment in the
perfection or priority of Secured Party's security interest in the Collateral.

          5.4    ATTACHMENT. (a) Any material portion of Grantor's assets is
attached, seized, levied on, or comes into possession of a trustee or receiver
and the attachment, seizure or levy is not removed in thirty (30) days; (b) a
judgment or other claim becomes a Lien on a

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material portion of Grantor's assets; or (c) a notice of lien, levy, or
assessment is filed against a material portion of Grantor's assets by any
government agency and not paid or contested in good faith in compliance with all
appropriate procedures within thirty (30) days after Grantor receives notice.
These are not Events of Default if stayed or if a bond is posted pending contest
by Grantor.

          5.5    MISREPRESENTATIONS. If Grantor or any Person acting for Grantor
makes any material misrepresentation or material misstatement now or later in
any warranty or representation in this Agreement or in any communication
delivered to Secured Party or to induce Secured Party to enter this Agreement.

          5.6    SECURED PARTY'S RIGHTS AND REMEDIES

          5.7    RIGHTS AND REMEDIES. Subject to the subordination provisions
contained in the Intercreditor Agreement and Article 4 of the Indenture, upon
the occurrence and during the continuance of an Event of Default, Secured Party
may, at its election, without notice of its election and without demand, do any
one or more of the following, all of which are authorized by Grantor:

                 (a)  Exercise all rights available to it under the New York
Uniform Commercial Code and applicable law;

                 (b)  Set off and apply to the obligations any and all (a)
balances and deposits of Grantor held by Secured Party, or (b) indebtedness at
any time owing to or for the credit or the account of Grantor held by Secured
Party; and

                 (c)  Sell the Collateral at either a public or private sale, or
both, by way of one or more contracts or transactions, for cash or on terms, in
such manner and at such places (including Grantor's premises) as Secured Party
reasonably determines is commercially reasonable.

          5.8    REMEDIES CUMULATIVE. Secured Party's rights and remedies under
this Agreement, and all other agreements shall be cumulative. Secured Party
shall have all other rights and remedies not inconsistent herewith as provided
under the New York Uniform Commercial Code, by law, or in equity. No exercise by
Secured Party of one right or remedy shall be deemed an election, and no waiver
by Secured Party of any Event of Default on Grantor's part shall be deemed a
continuing waiver. No delay by Secured Party shall constitute a waiver,
election, or acquiescence by it.

          5.9    DEMAND; PROTEST. Except as set forth herein, Grantor waives
demand, protest, notice of protest, notice of default or dishonor, notice of
payment and nonpayment, notice of any default, nonpayment at maturity, release,
compromise, settlement, extension, or renewal of accounts, documents,
instruments, chattel paper, and guarantees at any time held by Secured Party on
which Grantor may in any way be liable.

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     6.   CHOICE OF LAW AND VENUE; JURY TRIAL WAIVER

          The laws of the State of New York shall apply to this Agreement.
GRANTOR ACCEPTS FOR ITSELF AND IN CONNECTION WITH ITS PROPERTIES,
UNCONDITIONALLY, THE NON-EXCLUSIVE JURISDICTION OF ANY STATE OR FEDERAL COURT OF
COMPETENT JURISDICTION IN THE STATE OF NEW YORK IN ANY ACTION, SUIT, OR
PROCEEDING OF ANY KIND, AGAINST IT WHICH ARISES OUT OF OR BY REASON OF THIS
AGREEMENT; PROVIDED, HOWEVER, THAT IF FOR ANY REASON SECURED PARTY CANNOT AVAIL
ITSELF OF THE COURTS OF THE STATE OF NEW YORK, GRANTOR ACCEPTS JURISDICTION OF
THE COURTS AND VENUE IN THE COUNTY OF NEW YORK, NEW YORK. NOTWITHSTANDING THE
FOREGOING, THE SECURED PARTY SHALL HAVE THE RIGHT TO BRING ANY ACTION OR
PROCEEDING AGAINST THE BORROWER OR ITS PROPERTY IN THE COURTS OR ANY OTHER
JURISDICTION WHICH THE SECURED PARTY REASONABLY DEEMS NECESSARY OR APPROPRIATE
IN ORDER TO REALIZE ON THE COLLATERAL OR TO OTHERWISE ENFORCE THE SECURED
PARTY'S RIGHT AGAINST THE GRANTOR OR ITS PROPERTY.

          GRANTOR AND SECURED PARTY EACH HEREBY WAIVE THEIR RESPECTIVE RIGHTS TO
A JURY TRIAL OF ANY CLAIM OR CAUSE OF ACTION BASED UPON OR ARISING OUT OF ANY OF
THIS AGREEMENT OR ANY OF THE TRANSACTIONS CONTEMPLATED HEREIN, INCLUDING
CONTRACT CLAIMS, TORT CLAIMS, BREACH OF DUTY CLAIMS, AND ALL OTHER COMMON LAW OR
STATUTORY CLAIMS. EACH PARTY RECOGNIZES AND AGREES THAT THE FOREGOING WAIVER
CONSTITUTES A MATERIAL INDUCEMENT FOR IT TO ENTER INTO THIS AGREEMENT. EACH
PARTY REPRESENTS AND WARRANTS THAT IT HAS REVIEWED THIS WAIVER WITH ITS LEGAL
COUNSEL AND THAT IT KNOWINGLY AND VOLUNTARILY WAIVES ITS JURY TRIAL RIGHTS
FOLLOWING CONSULTATION WITH LEGAL COUNSEL.

     7.   GENERAL PROVISIONS

          7.1    SUCCESSORS AND ASSIGNS. This Agreement shall bind and inure to
the benefit of the respective successors and permitted assigns of each of the
parties; PROVIDED, HOWEVER, that neither this Agreement nor any rights hereunder
may be assigned by Grantor without Secured Party's prior written consent, which
consent may be granted or withheld in Secured Party's reasonable discretion.
Secured Party shall have the right without the consent of or notice to Grantor
to sell, transfer, negotiate, or grant participations in all or any part of, or
any interest in Secured Party's obligations, rights and benefits hereunder.

          7.2    INDEMNIFICATION. Grantor shall defend, indemnify and hold
harmless Secured Party and its officers, employees, and agents against: (a) all
loss, damage, obligations, demands, claims, liabilities and expenses claimed or
asserted by the Grantor or by any other party in connection with the
transactions contemplated by this Agreement; and (b) all reasonable losses or
Secured Party's expenses in any way suffered, incurred, or paid by Secured Party
as a result of or in any way arising out of, following, or consequential to
transactions between Secured Party and Grantor whether under this Agreement, or
otherwise (including without

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limitation reasonable attorneys' fees and expenses), except for losses caused by
Secured Party's gross negligence or willful misconduct.

          7.3    RIGHT OF SET-OFF. Grantor hereby grants to Secured Party, a
lien, security interest and right of setoff as security for all obligations and
liabilities to Secured Party, whether now existing or hereafter arising upon and
against all deposits, credits, collateral and property, now or hereafter in the
possession, custody, safekeeping or control of Secured Party or any entity under
the control of the Secured Party or in transit to any of them. At any time after
the occurrence and during the continuance of an Event of Default, but subject in
all cases to the Intercreditor Agreement, without demand or notice, Secured
Party may set off the same or any part thereof and apply the same to any
liability or obligation of Grantor even though unmatured and regardless of the
adequacy of any other collateral securing the loan. ANY AND ALL RIGHTS TO
REQUIRE SECURED PARTY TO EXERCISE ITS RIGHTS OR REMEDIES WITH RESPECT TO ANY
OTHER COLLATERAL WHICH SECURES THE LIABILITIES OF GRANTOR TO SECURED PARTY,
PRIOR TO EXERCISING ITS RIGHT OF SETOFF WITH RESPECT TO SUCH DEPOSITS, CREDITS
OR OTHER PROPERTY OF THE GRANTOR, ARE HEREBY KNOWINGLY, VOLUNTARILY AND
IRREVOCABLY WAIVED.

          7.4    TIME OF ESSENCE. Time is of the essence for the performance of
all obligations set forth in this Agreement.

          7.5    SEVERABILITY OF PROVISIONS. Each provision of this Agreement
shall be severable from every other provision of this Agreement for the purpose
of determining the legal enforceability of any specific provision.

          7.6    AMENDMENTS IN WRITING, INTEGRATION. This Agreement cannot be
changed or terminated orally. All prior agreements, understandings,
representations, warranties, and negotiations between the parties hereto with
respect to the subject matter of this Agreement, if any, are merged into this
Agreement.

          7.7    COUNTERPARTS. This Agreement may be executed in any number of
counterparts and by different parties on separate counterparts, each of which,
when executed and delivered, shall be deemed to be an original, and all of
which, when taken together, shall constitute but one and the same Agreement.

          7.8    SURVIVAL. All covenants, representations and warranties made in
this Agreement shall continue in full force and effect so long as any
liabilities and obligations under the Notes, the Indenture and this Agreement
remain outstanding. The obligations of Grantor to indemnify Secured Party with
respect to the expenses, damages, losses, costs and liabilities described in
this Agreement shall survive until all applicable statute of limitations periods
with respect to actions that may be brought against Secured Party have run.

          7.9    GRANTOR WAIVERS. Grantor waives any right to require Secured
Party to (a) proceed against any Person; (b) proceed against or exhaust any
security held from any Person; (c) marshal any assets of any Person; or (d)
pursue any other remedy in Secured Party's power whatsoever. Secured Party may,
at its election, exercise or decline or fail to exercise any right or

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remedy it may have against any Person or any security held by Secured Party,
including without limitation the right to foreclose upon any such security by
judicial or nonjudicial sale, without affecting or impairing in any way the
liability of Grantor hereunder. Grantor waives any defense arising by reason of
any disability or other defense of any Person or by reason of the cessation from
any cause whatsoever of the liability of any Person. Grantor waives any setoff,
defense or counterclaim that any Person may have against Secured Party. Grantor
waives any defense arising out of the absence, impairment or loss of any right
of reimbursement or subrogation or any other rights against any Person. Grantor
shall have no right of subrogation or reimbursement, contribution or other
rights against any Person, and Grantor waives any right to enforce any remedy
that Secured Party now has or may hereafter have against any Person. Grantor
waives all rights to participate in any security now or hereafter held by
Secured Party. Grantor waives all presentments, demands for performance, notices
of nonperformance, protests, notices of protest, notices of dishonor, and
notices of acceptance of this Agreement and of the existence, creation, or
incurring of new or additional indebtedness. Grantor assumes the responsibility
for being and keeping itself informed of the financial condition of any Person
and of all other circumstances bearing upon the risk of nonpayment of any
indebtedness or nonperformance of any obligation represented by the
Indebtedness, warrants to Secured Party that it will keep so informed, and
agrees that absent a request for particular information by Grantor, Secured
Party shall have no duty to advise Grantor of information known to Secured Party
regarding such condition or any such circumstances.

          7.10   INSOLVENCY. If any Person becomes insolvent or is adjudicated
bankrupt or files a petition for reorganization, arrangement, composition or
similar relief under any present or future provision of the United States
Bankruptcy Code, or if such a petition is filed against any Person, and in any
such proceeding some or all of any indebtedness or obligations under the
Indenture are terminated or rejected or any obligation of any Person modified or
abrogated, or if any Person's obligations are otherwise avoided for insolvency,
bankruptcy or any similar reason, Grantor agrees that Grantor's liability
hereunder shall not thereby be affected or modified and such liability shall
continue in full force and effect as if no such action or proceeding had
occurred. This Agreement shall continue to be effective or be reinstated, as the
case may be, if any payment must be returned by Secured Party upon the
insolvency, bankruptcy or reorganization of any Person, as though such payment
had not been made.

          7.11   RATIFICATION OF IP AGREEMENT. Grantor hereby ratifies,
confirms, and reaffirms, all and singular, the terms and conditions of a certain
Intellectual Property Security Agreement dated June 18, 2004 by and between
Grantor and Secured Party (the "IP Agreement") and acknowledges, confirms and
agrees that the IP Agreement contains an accurate and complete listing of all
Intellectual Property Collateral (as defined therein) of Grantor and such IP
Agreement secures all Indebtedness.

          7.12   INDENTURE CONTROLLING. In the event and to the extent of any
inconsistency or conflict between the provisions of this Agreement and those
contained in the Indenture, the provisions of the Indenture shall control.

                            [SIGNATURE PAGE FOLLOWS]

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

                                  ("Grantor")
                                  IBASIS HOLDINGS, INC.
                                  By: /s/ Ofer Gneezy
                                      ------------------------------------------
                                  Name: Ofer Gneezy
                                        ----------------------------------------
                                  Title: President and Chief Executive Officer
                                         ---------------------------------------


          [SIGNATURE PAGE TO iBASIS HOLDINGS, INC. SECURITY AGREEMENT]

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                                  ("Secured Party")
                                  THE BANK OF NEW YORK
                                  By: /s/ Geovanni Barris
                                      ------------------------------------------
                                  Name: Geovanni Barris
                                        ----------------------------------------
                                  Title: Vice President
                                         ---------------------------------------


          [SIGNATURE PAGE TO iBASIS HOLDINGS, INC. SECURITY AGREEMENT]

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                                    EXHIBIT A

          The Collateral shall consist of all right, title and interest of
Grantor in and to the following:

          All assets, including without limitation, all goods, equipment,
inventory, contract rights or rights to payment of money, leases, license
agreements, franchise agreements, general intangibles (including payment
intangibles), accounts (including health-care receivables), documents,
instruments (including any promissory notes), chattel paper (whether tangible or
electronic), cash, deposit accounts, fixtures, letters of credit rights (whether
or not the letter of credit is evidenced by a writing), commercial tort claims,
securities and all other investment property, supporting obligations, and
financial assets, whether now owned or hereafter acquired, wherever located; and

          All Grantor's books relating to the foregoing and any and all claims,
rights and interests in any of the above and all substitutions for, additions,
attachments, accessories, accessions and improvements to and replacements,
products, proceeds and insurance proceeds of any or all of the foregoing.

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                                   SCHEDULE 1

         IDENTIFICATION OF ORGANIZATION, JURISDICTION, ETC., OF GRANTOR

Legal name: iBasis Holdings, Inc.

Type of organization: Corporation

Jurisdiction of incorporation: Delaware

Employer identification number:

Place of business: 20 Second Avenue, Burlington, MA 01803