FOURTH LOAN MODIFICATION AGREEMENT

Exhibit 10.1

FOURTH LOAN MODIFICATION AGREEMENT

This Fourth Loan Modification Agreement (this “Loan Modification Agreement’) is entered into as of September 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 W 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, a certain Second Loan Modification Agreement entered into as of February 27, 2003, and a certain Third Loan Modification Agreement dated as of June, 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 deleting 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 (1^st) Business Day after the expiration of each Permitted Overadvance Period.”

and inserting lieu thereof the following:

“plus

(iii) up to $5,000,000.00 (the “Permitted Overadvance”) for the period commencing September 30, 2003 through October 9, 2003; provided the Permitted Overadvance must be repaid in full on October 10, 2003.”

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 July 31, 2003 - $22,900,000.00

(E)                                 from August 1, 2003 through August 31, 2003 - $18,000,000.00

(F)                                 from September 1, 2003 through September 30, 2003 - $16,750,000.00

(G)                                from October 1, 2003 through December 31, 2003 - - $20,100,000.00

(H)                               from January 1, 2004 through August 31, 2004 - $19,800,000.00

(I)                                    from September 1, 2004 through November 30, 2004 - $22,300,000.00

(J)                                   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:

“b. Minimum Cash or Excess Availability:

Borrower shall at all times maintain $5,000,000.00 in (i) cash deposits maintained at Silicon, and/or (ii) excess “availability” under this Agreement (net of Loans, Letters of Credit or other indebtedness under this Agreement), as determined by Silicon based upon the Credit Limit restrictions set forth in Section 1 above).”

D.                                    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 July 31, 2003 - 0.85 to 1.0

(E)                                 from August 1, 2003 through September 30, 2003 - - 0.75 to 1.0

(F)                                 from October 1, 2003 through December 31, 2003 - - 0.80 to 1.0

(G)                                from January 1, 2004 through March 31, 2004 - 0.85 to 1.0

(H)                               from April 1, 2004 through June 30, 2004 - 0.90 to 1.0

(I)                                    from July 1, 2004 through September 30, 2004 - 0.95 to 1.0

(J)                                   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 July 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.

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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.

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.

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.

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