Having used IP Telephony for a number of years I need a workable SIP client for all of the devices I use. I have found a very capable client for my Android phones (SipDroid) and on Windows/MacOSX I generally use X-Lite (as well as it’s paid version EyePhone) from Counterpath.
Having tried several different Linux SIP clients (Ekiga, Twinkle, …) but all discarded them because of weird UI’s and/or problems with stability I noticed that there is a Linux version of XLite available. Unfortunately on current versions of Ubuntu (10.04) it needs a deprecated version of a library.
Download XLite Linux: http://www.counterpath.com/x-lite-3.0-for-linux-download.html
Dowload libstdc++.so.5: http://packages.debian.org/lenny/i386/libstdc++5/download
[you will need root permissions for all of the below]
Enjoy calling from your Ubuntu machine !
EDIT: Since upgrading to Ubuntu 11 (Natty) I have switched to QuteCom