I tried to use it for SIP recently and it did not work well for me at all. I am using Arch Linux and a tiling window manager. It doesn't seem to cooperate with my fairly normal sound setup and the interface is messed up on awesomewm at least. Not worth the effort.

The most reliable VoIP program I've found is twinkle, but it hasn't been updated in years afaik. I used Ekiga exclusively for SIP and video chat for several months but it was plagued with really annoying bugs, including frequent hard locks and unreliable presence/call functionalities.

I haven't been impressed with any Linux SIP/video chat applications to be honest. Empathy and Pidgin use gstreamer which usually doesn't work very well either. The most reliable video chat experience I've had is Skype, despite their buggy Linux client.