Here’s some SIP clients I’ve found that work well in linux.
Zoiper
A new one I came across just recently, works great, very easy to configure and use.
Has very good voice quality, and works well, but has an odd interface, which makes it harder to configure.
Comes preinstalled on Ubuntu Linux, and works ok, but I’ve had problems with the G711 ulaw/alaw codec with it, voice quality seems to drop quickly with this codec.