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                                                                     Exhibit 5



                              Bingham McCutchen LLP
                               150 Federal Street
                           Boston, Massachusetts 02110
                                tel: 617-951-8000
                                fax: 617-951-8736


August 18, 2004

iBasis, Inc.
iBasis Global, Inc.
iBasis Holdings, Inc.
iBasis Securities Corporation
20 Second Avenue
Burlington, MA 01803


         RE: REGISTRATION STATEMENT ON FORM S-1 UNDER THE SECURITIES ACT OF 1933


Dear Ladies and Gentlemen:

         We have acted as counsel to (i) iBasis, Inc., a Delaware corporation
(the "COMPANY"), (ii) iBasis Global, Inc., a Delaware corporation ("GLOBAL"),
(iii) iBasis Holdings, Inc., a Delaware corporation ("HOLDINGS"), and (iv)
iBasis Securities Corporation, a Massachusetts corporation ("SECURITIES" and
together with Global and Holdings, the "SUBSIDIARY GUARANTORS"), in connection
with the registration under the Securities Act of 1933, as amended (the "ACT"),
of (A) $29 million aggregate principal amount of the Company's 8% Secured
Convertible Notes due 2007 (the "NEW SECURED NOTES"), (B) Guarantees (the
"GUARANTEES") of the New Secured Notes by the Subsidiary Guarantors and (C)
20,961,971 shares of the Company's common stock, $0.001 par value per share (the
"COMMON STOCK"), including (1) 15,675,675 shares of Common Stock issuable upon
conversion of the New Secured Notes, plus such indeterminate number of shares of
Common Stock that may be issuable as a result of adjustments of the conversion
price of the New Secured Notes (the "NEW SECURED NOTE SHARES"), (2) 5,176,065
shares of Common Stock (the "WARRANT SHARES") issuable upon conversion of
certain warrants (the "WARRANTS"), dated as of June 18, 2004, and (3) 110,231
other previously-issued shares of Common Stock (the "OTHER SHARES"), pursuant to
a Registration Statement on Form S-1 (as amended, the "REGISTRATION STATEMENT"),
initially filed with the Securities and Exchange Commission on July 13, 2004.

         The New Secured Notes and the Guarantees were issued pursuant to an
Indenture (the "INDENTURE"), dated as of June 18, 2004, by and among the
Company, the Subsidiary Guarantors and The Bank of New York, as the Trustee
thereunder (the "TRUSTEE"). Capitalized terms used herein and not defined shall
have the respective meanings ascribed to them in the Indenture.

         In connection with this opinion, we have examined originals or copies
of: the Registration Statement; the Indenture; the Global Note, issued by the
Company on June 18, 2004, representing $29.0 million aggregate principal amount
of the New Secured Notes; the Guarantees; the form of Warrant Certificate,
issued by the Company on June 18, 2004, representing the Warrants; resolutions
duly adopted by the board of directors of each of the Company and the Subsidiary
Guarantors; and certified copies of the certificate of incorporation or articles
of organization of each of the Company and the Subsidiary Guarantors, each as
amended to date. In addition, we have examined such other


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corporate and public records and agreements, instruments, certificates and other
documents as we have deemed necessary or appropriate for purposes of this
opinion. The Indenture, the Guarantees and the New Secured Notes are referred to
herein collectively as the "TRANSACTION DOCUMENTS."

         We have assumed the genuineness of all signatures, the conformity to
the originals of all documents reviewed by us as copies, the authenticity and
completeness of all original documents reviewed by us in original or copy form
and the legal competence of each individual executing any document.

         For purposes of this opinion, we have made such examination of law as
we have deemed necessary. This opinion is limited solely to the internal
substantive laws of the State of New York as applied by courts located in New
York; with respect to Securities, the Massachusetts Business Corporation Law as
applied by courts located in Massachusetts; the federal laws of the United
States of America (except for tax, antitrust, blue sky and securities laws, as
to which we express no opinion); and the Delaware General Corporation Law, as
applied by courts located in Delaware, and the applicable provisions of the
Delaware Constitution and the reported judicial decisions interpreting those
laws; and we express no opinion as to the laws of any other jurisdiction.

         Our opinion is further subject to the following exceptions,
qualifications and assumptions, all of which we understand to be acceptable to
you:

         (a)      The enforcement of any obligations of, or any security
                  interest granted by, the Company, the Subsidiary Guarantors or
                  any other Person, whether under any of the Transaction
                  Documents or otherwise, may be limited by bankruptcy,
                  insolvency, reorganization, moratorium, marshaling or other
                  laws and rules of law affecting the enforcement generally of
                  creditors' rights and remedies, including, without limitation,
                  fraudulent conveyance and fraudulent transfer laws (including
                  such as may deny giving effect to waivers of debtors' or
                  guarantors' rights); and we express no opinion as to the
                  status under any fraudulent conveyance laws or fraudulent
                  transfer laws of any of the obligations of, or any security
                  interest granted by, any of the Company, the Subsidiary
                  Guarantors, or any other Person, whether under any of the
                  Transaction Documents or otherwise, or to the applicable law
                  with respect to such matters.

         (b)      The enforcement of the rights of any Person may in all cases
                  be subject to an implied duty of good faith and fair dealing
                  and to general principles of equity (regardless of whether
                  such enforceability is considered in a proceeding at law or in
                  equity), including, without limitation, concepts of
                  materiality and reasonableness.

         (c)      We express no opinion as to the effect of events occurring,
                  circumstances arising, or changes of law becoming effective or
                  occurring, after the date hereof on the matters addressed in
                  this opinion letter, and we assume no responsibility to inform
                  you of additional or changed facts, or changes in law, of
                  which we may become aware.

         (d)      We have assumed that each of the Transaction Documents to
                  which each of the Subsidiary Guarantors is a party is
                  necessary or convenient to the conduct, promotion or
                  attainment of the business of such Subsidiary Guarantor.


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         (e)      We have assumed that the Company will have a sufficient number
                  of authorized but unissued shares of Common Stock available
                  under its certificate of incorporation to accommodate the
                  issuance of the New Secured Note Shares and the Warrant
                  Shares.

         Based upon the foregoing, and subject to the limitations and
qualifications set forth above, we are of the opinion that:

         1. Each of the New Secured Notes and the Guarantees to which the
Company and the Subsidiary Guarantors are party is a valid and binding
obligation of the Company and/or each such Subsidiary Guarantor, enforceable
against the Company and each such Subsidiary Guarantor in accordance with its
respective terms.

         2. The New Secured Note Shares and the Warrant Shares have been duly
authorized by the Company, and, when issued in accordance with the terms of the
New Secured Notes and the Warrants, will be validly issued, fully paid and
nonassessable.

         3. The Other Shares have been duly authorized and validly issued by the
Company, and are fully paid and nonassessable.

         We consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the Registration Statement and the
reference to us under the heading "Legal Matters" in the related prospectus.


Very truly yours,

/s/ BINGHAM McCUTCHEN LLP
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BINGHAM McCUTCHEN LLP


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