Talkonaut

Talkonaut is a Jabber (XMPP) client with peer-to-peer voice calling over Jingle Audio extension. Talkonaut can be used with any Jabber account. Talkonaut supports a number of XMPP extensions including multi-user conference chat (XEP-0045), vCard based avatars (XEP-0153) and many others.

You can also use Talkonaut to make cheap voice calls to landline or mobile phone numbers through GTalk2VoIP infrastructure.