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 (Realtime 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.