<PAGE>
IBASIS, INC.
20 Second Ave
Burlington, Massachusetts 01803
SUPPLEMENT TO PROSPECTUS DATED MARCH 9, 2000
with respect to
3,500,000 Shares of Common Stock par value $0.001
The date of this Supplement is April 11, 2000
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Reference is made to our prospectus, dated March 9, 2000, relating to
our offering of 2,026,637 shares of common stock and 1,473,363 shares of common
stock offered by certain selling stockholders.
We have amended note (1) to the table of selling stockholders included
in the section entitled "Principal and Selling Stockholders," and the disclosure
in the section entitled "Underwriting" to reflect the sale by certain
stockholders upon the exercise of the underwriters' over-allotment option who
have been substituted for those stockholders originally listed in our
prospectus.
Therefore, note (1) to the table of selling stockholders included in
the section entitled "Principal and Selling Stockholders" on page 62 is hereby
restated in its entirety to read as follows:
"(1) Assumes no exercise of the underwriters' over-allotment option. If the
underwriters exercise their over-allotment option, each of the following
selling stockholders may sell up to the additional number of shares set
forth below:
<TABLE>
<CAPTION>
SHARES SHARES
BENEFICIALLY OWNED BENEFICIALLY OWNED
PRIOR TO ASSUMED ADDITIONAL AFTER ASSUMED
EXERCISE OF SHARES TO EXERCISE OF
OVER-ALLOTMENT OPTION BE OFFERED OVER-ALLOTMENT OPTION
--------------------- ---------- ----------------------
NAME OF BENEFICIAL OWNER NUMBER PERCENT NUMBER PERCENT
------------------------ --------- --------- -------- ---------
<S> <C> <C> <C> <C> <C>
Seruus Telecom Fund, L.P. .......... 999,999 3.2% 100,000 899,999 2.7%
Charles S. Houser (10).............. 1,099,999 3.5% 10,000 989,999 2.9%
</TABLE>
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In addition, the fifth paragraph on page 69 under the caption
"Underwriting" is hereby restated in its entirety to read as follows:
"In the event that the underwriters do not exercise their
over-allotment option in full, Charles S. Houser and Seruus Telecom Fund, L.P.
shall sell up to 10,000 and 100,000 shares, respectively, and any additional
shares shall be sold by iBasis. In the event that the over-allotment option is
exercised for less than 110,000 shares, such shares shall be distributed between
the selling stockholders pro rata based on their proportional share allotment."
