liblinphone

This library includes ALL the features of linphone. It is designed to be easy to use, directly from a user interface.
Its API is described in coreapi/linphonecore.h .
Making a SIP call is as easy as:
/* first create a LinphoneCore object: this initialize your virtual SIP phone*/
/* note: vtable is a structure that contains callbacks you must implement to get notified
of various kind of information*/
LinphoneCore *lc=linphone_core_new(&vtable,".myconfig",NULL);
/* initiate an outgoing call*/
linphone_core_invite(lc,"sip:alice@example.net");
...
/* and periodically (ex: every 100ms) call this function to make the linphone engine to work:*/
linphone_core_iterate(lc);

/*terminate this call*/
linphone_core_terminate(lc,"sip:alice@example.net");

There are various methods to create proxy settings, send instant messaging, get presence information...

This API is not yet stable and is subject to changes with new releases.