If you like to know exactly what your VoIP is doing-i.e. you want access to the source code-you might like this list of 50 different applications that do VoIP, but are open source. There are some I knew in here, and many I did not.

Of course, the problem with all these choices is: which one do you choose? That is a question I have a hard time answering. It's a shame this list classified YATE as an H.323 client as it really so much more. I had some conversations with one of the developers several years ago and while I had a hard time figuring out how to make it work for me, I was impressed with how much it could do with so little resources!

In any case, if there are other open-source VoIP applications you want to highlight, leave them in the comments!