EXHIBIT 10.1

THIRD LOAN MODIFICATION AGREEMENT

        This Third Loan Modification Agreement (this “Loan Modification
Agreement”) is entered into as of June 30, 2003, by and between SILICON VALLEY
BANK, a California-chartered bank, with its principal place of business at 3003
Tasman Drive, Santa Clara, California 95054 and with a loan production office
located at One Newton Executive Park, Suite 200, 2221 Washington Street, Newton,
Massachusetts 02462, doing business under the name “Silicon Valley East”(“Bank”)
and (i) IBASIS, INC., a Delaware corporation with its chief executive office
located at 20 Second Avenue, Burlington, Massachusetts 01803 and (ii) IBASIS
GLOBAL, INC., Delaware corporation with its chief executive office located at 20
Second Avenue, Burlington, Massachusetts 01803 (jointly and severally,
individually and collectively, “Borrower”).

1.     DESCRIPTION OF EXISTING INDEBTEDNESS AND OBLIGATIONS. Among other
indebtedness and obligations which may be owing by Borrower to Bank, Borrower is
indebted to Bank pursuant to a loan arrangement dated as of December 30, 2002,
evidenced by, among other documents, a certain Loan and Security Agreement dated
as of December 30, 2002 between Borrower and Bank, as amended by a certain First
Loan Modification Agreement dated as of January 30, 2003 and a certain Second
Loan Modification Agreement entered into as of February 27, 2003 (as amended,
the “Loan Agreement”). Capitalized terms used but not otherwise defined herein
shall have the same meaning as in the Loan Agreement.

2.     DESCRIPTION OF COLLATERAL. Repayment of the Obligations is secured by the
Collateral as described in the Loan Agreement and certain Intellectual Property
Security Agreements each dated December 30, 2002 (together with any other
collateral security granted to Bank, the “Security Documents”).

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Obligations shall be referred to as the “Existing
Loan Documents”.

3.     DESCRIPTION OF CHANGE IN TERMS.

  Modification to Loan Agreement.

  A. The Loan Agreement shall be amended by adding the following text at the end
of Section 1(B) of the Schedule thereto:

  “plus

  (iii) up to $5,000,000.00 (the “Permitted Overadvance”) for each of the
following seven Business Day periods (each a “Permitted Overadvance Period”):
(a) June 30 through the next six (6) Business Days of each year, (b) September
30 through the next six (6) Business Days of each year, (c) December 31 though
the next six (6) Business Days of each year, and (d) March 31 through the next
six (6) Business Days of each year; provided in each instance the Permitted
Overadvance must be repaid in full on the first (1st) Business Day after the
expiration of each Permitted Overadvance Period.

  B. The Loan Agreement shall be amended by deleting Section 5(a) of the
Schedule thereto and inserting in lieu thereof the following:

  “a. Minimum Tangible Net Worth:

  Borrower shall maintain a Tangible Net Worth of not less than:

  (A) from the date of this Agreement through March 31, 2003 — $33,800,000.00

  (B) from April 1, 2003 through May 31, 2003 — $27,100,000.00

  (C) from June 1, 2003 through June 30, 2003 — $22,750,000.00

  (D) from July 1, 2003 through September 30, 2003 — $22,900,000.00

  (E) from October 1, 2003 through December 31, 2003 — $20,100,000.00

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  (F) from January 1, 2004 through August 31, 2004 — $19,800,000.00

  (G) from September 1, 2004 through November 30, 2004 — $22,300,000.00

  (H) from December 1, 2004 and thereafter — $26,300,000.00”

  C. The Loan Agreement shall be amended by deleting Section 5(c) of the
Schedule thereto in its entirety and inserting in lieu thereof the following:

  “c. Quick Ratio.

  Borrower shall at all times maintain a ratio of Quick Assets to Current
Liabilities of at least:

  (A) from the date of this Agreement through March 31, 2003 — 0.95 to 1.0

  (B) from April 1, 2003 through May 31, 2003 — 0.90 to 1.0

  (C) from June 1, 2003 through June 30, 2003 — 0.80 to 1.0

  (D) from July 1, 2003 through September 30, 2003 — 0.85 to 1.0

  (E) from October 1, 2003 through December 31, 2003 — 0.80 to 1.0

  (F) from January 1, 2004 through March 31, 2004 — 0.85 to 1.0

  (G) from April 1, 2004 through June 30, 2004 — 0.90 to 1.0

  (H) from July 1, 2004 through September 30, 2004 — 0.95 to 1.0

  (I) from October 1, 2004 and thereafter — 1.0 to 1.0”

4.     FEES. Borrower shall reimburse Bank for all reasonable legal fees and
expenses incurred in connection with this amendment to the Existing Loan
Documents.

5.     WAIVER. The Bank hereby waives the Borrower’s failure to comply with the
“Minimum Tangible Net Worth” covenant set forth in Section 5(a) of the Schedule
to the Loan Agreement for the period ending May 31, 2003. The Bank’s waiver of
Borrower’s compliance with the foregoing covenant shall apply only to the
foregoing specified period.

6.     CONSISTENT CHANGES. The Existing Loan Documents are hereby amended
wherever necessary to reflect the changes described above.

7.     RATIFICATION OF LOAN DOCUMENTS. Borrower hereby ratifies, confirms, and
reaffirms all terms and conditions of all security or other collateral granted
to the Bank, and confirms that the indebtedness secured thereby includes,
without limitation, the Obligations.

8.     RATIFICATION OF INTELLECTUAL PROPERTY SECURITY AGREEMENTS. Borrower
hereby ratifies, confirms, and reaffirms, all and singular, the terms and
conditions of the IP Agreements and acknowledges, confirms and agrees that the
IP Agreements contain an accurate and complete listing of all Intellectual
Property.

9.     RATIFICATION OF PERFECTION CERTIFICATE. Borrower hereby ratifies,
confirms, and reaffirms, all and singular, the terms and disclosures contained
in certain Perfection Certificates delivered to the Bank on or about December
30, 2002, and acknowledges, confirms and agrees the disclosures and information
provided therein has not changes, as of the date hereof.

10.     NO DEFENSES OF BORROWER. Borrower hereby acknowledges and agrees that
Borrower has no offsets, defenses, claims, or counterclaims against the Bank
with respect to the Obligations, or otherwise, and that if Borrower now has, or
ever did have, any offsets, defenses, claims, or counterclaims against the Bank,
whether known or unknown, at law or in equity, all of them are hereby expressly
WAIVED and Borrower hereby RELEASES the Bank from any liability thereunder.

11.     CONTINUING VALIDITY. Borrower understands and agrees that in modifying
the existing Obligations, Bank is relying upon Borrower’s representations,
warranties, and agreements, as set forth in the Existing Loan Documents. Except
as expressly modified pursuant to this Loan Modification Agreement, the terms of
the Existing Loan Documents remain unchanged and in full force and effect.
Bank’s agreement to modifications to the existing Obligations pursuant to this
Loan Modification Agreement in no way shall obligate Bank to make any future
modifications to the Obligations. Nothing in this Loan Modification Agreement
shall constitute a satisfaction of the Obligations. It is the intention of Bank
and Borrower to retain as liable parties all makers of Existing Loan Documents,
unless the party is expressly released by Bank in writing. No maker will be
released by virtue of this Loan Modification Agreement.

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12.     COUNTERSIGNATURE. This Loan Modification Agreement shall become
effective only when it shall have been executed by Borrower and Bank.

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        This Loan Modification Agreement is executed as a sealed instrument
under the laws of the Commonwealth of Massachusetts.

BORROWER:

IBASIS, INC.

By: /s/ Ofer Gneezy
      ——————————————
Name: Ofer Gneezy
Title: Chief Executive Officer

IBASIS GLOBAL, INC.

By: /s/ Richard Tennant
      ——————————————
Name: Richard Tennant
Title: Chief Financial Officer

BANK:

SILICON VALLEY BANK, d/b/a
SILICON VALLEY EAST

By: /s/ Nancy E. Funkhouser
      ——————————————
Name: Nancy E. Funkhouser
Title: Vice President

        The undersigned ratifies, confirms and reaffirms, all and singular, the
terms and conditions of a certain Unconditional Guaranty dated December 30, 2002
(the “Guaranty”) and a certain Security Agreement dated December 30, 2002 (the
“Security Agreement”) and acknowledges, confirms and agrees that the Guaranty
and the Security Agreement shall remain in full force and effect and shall in no
way be limited by the execution of this Loan Modification Agreement, or any
other documents, instruments and/or agreements executed and/or delivered in
connection herewith.

IBASIS SECURITIES CORPORATION

By:             /s/ Ofer Gneezy
            ——————————————
Name:       Ofer Gneezy
            ——————————————
Title:         Chief Executive Officer
            ——————————————

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