

	Concert Global Clearinghouse and iBasis Announce Interconnection of their Internet Telephony Networks Leaders in VoIP To Exchange International Voice and Fax Traffic, Leverage Combined Footprint to Broaden Global Reach Burlington, Mass.-May 15, 2000-Concert, the AT&T/BT global communications venture, and iBasis, Inc., (Nasdaq: IBAS), leaders in advanced Internet-based communications, today announced an agreement that will enable the two companies to exchange international voice and fax traffic as well as leverage a combined VoIP (Voice over Internet-Protocol) footprint to service new markets and broaden their networks' global reach. This agreement will result in seamless connectivity between Concert Global Clearinghouse's service provider members and The iBasis Network™, the world's largest international Cisco Powered Network™ for Internet telephony. Both iBasis and the Concert Global Clearinghouse provide traditional carriers, ISPs and next-gen telcos comprehensive solutions that enable them to quickly and efficiently extend their global footprint and expand their international services. "Interconnecting our network to The iBasis Network creates an IP-based communications service of unmatched capacity, quality and global reach," said Cathy Martine, President, International Carrier Services, Concert. "As the demand for Internet telecommunications services continues to grow, this relationship between two of the leading VoIP carriers represents increased business opportunity and greater choice to both Concert's and iBasis' wholesale customers." "Adding Concert as a strategic partner further extends our leadership as the VoIP provider of choice for many of the world's largest carriers," said Ofer Gneezy, president and CEO, iBasis. "Concert's network is an excellent complement to our own extensive global footprint and combined with the traffic exchange established by this agreement significantly enhances the value of both networks and reinforces our position as co-leaders of the burgeoning Internet telephony market." The Concert IP Network The Concert Global Clearinghouse is supported by Concert's new, high speed, global Internet Protocol backbone network, which offers a robust communications platform for Global Clearinghouse members. Members can connect directly to the IP backbone from 21 cities in 17 countries, enabling them to deliver faster and higher quality Internet communications services to their customers. When the Concert network is fully integrated with the AT&T Network Services (AGNS) network, within the next 12-15 months, the Concert IP backbone will extend to more than 60 cities worldwide. The iBasis Network The iBasis Network is a high capacity, carrier-class VoIP infrastructure that provides toll quality (the industry term for the highest level of service quality obtainable) international voice and fax service for more than 70 international carriers, including 11 of the top 12 US international carriers, into virtually every region of the world, as well as advanced hosted services, such as VoCore. About The Concert Global Clearinghouse (www.concert.com/clearinghouse) Formerly known as AT&T Global Clearinghouse, the Concert Global Clearinghouse is now being offered as a premier product of Concert, the AT&T/BT global communications venture. The Global Clearinghouse offers complete Internet communications service solutions for ISPs, ITSPs, carriers and next generation telcos, enabling them to offer a full range of services, including phone-to-phone voice and fax services and other value-added applications. The Global Clearinghouse acts as an intermediary for the financial settlement of Internet telephony traffic and guarantees payment to all members. It is the fastest growing clearinghouse,* and offers termination in 220 countries through its member network. About Concert [www.concert.com] Concert combines the innovation, creativity and daring of a start-up with the strengths and experience of its market leading parents-AT&T and BT. Designed and built from the ground up to serve the needs of multinational businesses, international carriers and Internet Service Providers, the company is committed to making global communications seamless, reliable and hassle-free for customers. The company is built on a foundation of unparalleled network capabilities, unprecedented global reach, and unmatched technology pedigree. About iBasis Founded in 1996, iBasis (Nasdaq: IBAS) is the leader in advanced Internet-based communications. iBasis has built the world's first global IP network that delivers toll quality voice and fax service and other advanced hosted communications solutions, including VoCore, to global tier one carriers and other service providers. The iBasis NetworkTM is the world's largest international Cisco Powered Network for Internet Telephony. More than 70 international carriers are iBasis customers, including 11 of the top 12 US international carriers. The company can be reached at its worldwide headquarters in Burlington, Mass., USA at 781-505-7500 or on the Internet at www.ibasis.net. iBasis has a long-standing strategic relationship with Cisco, as an alliance partner, an ecosystem partner, and one of only 21 Cisco service provider global accounts. Except for historical information, all of the expectations, projections and assumptions contained in the foregoing press release constitute forward-looking statements under the Private Securities Litigation Reform Act of 1995 and involve risks and uncertainties. Important factors that could cause actual results to differ materially from such forward-looking statements include, but are not limited to, the considerations described as "Risk Factors" in iBasis' prospectuses dated March 9, 2000, and the company's other SEC filings. ### iBasis, The iBasis Network and Assured Quality Routing are registered trademarks of iBasis, Inc. Cisco and Cisco Powered Network are registered trademarks of Cisco Systems, Inc * "IP Telephony Clearinghouses - A New Business Model". Published November 1999 by iLocus. iLocus, a division of Cape Saffron Ltd., is a UK-based research group focused on the IP telephony industry. http://www.ilocus.com. The Concert Global Clearinghouse has had more than a 20% increase in new members since December 1999, with a current membership of more almost 100 carriers and ISPs.