Linphone is a Web phone with a Qt interface.
It lets you make two-party calls over IP networks such as the Internet.
It uses the IETF protocols SIP (Session Initiation Protocol) and RTP (Real-time Transport Protocol) to make calls, so it should be able to communicate with other SIP-based Web phones. With several codecs available, it can be used with high speed connections as well as 28k modems.