] and Opal
] are both free, open source and written in C/C++
Iirc, Opal is based off OpenH323 but has a better feature support.
There is a few samples on how to use these included in the source code download, but there is also Ekiga
] for Linux and MyPhone
] for Windows which both implement most features available in the libraries.
You will need to get a few other open source libraries and tool to compile them however.
It is all explained in the readme/help files included with the source code