Sip-communicator For Ubuntu/Debian
Sip communicator is an audio/video phone and instant messenger that supports some of the most popular instant messaging and telephony protocols such as sip, jabber, AIM/ICQ, MSN. Yahoo Messenger, Bonjour, IRC, RCC and counting.
Add this PPA:
deb http://download.sip-communicator.org/deb unstable/
Then go to a terminal and type the following commands:
apt-get install sip-communicator-keyring
apt-get install sip-communicator
Go into Applications -> Internet -> SIP Communicator