Karmic Koala replaces Pidgin and Ekiga with Empathy. To chat with IM (Yahoo, Gadu-gadu, Google Talk, etc) you can use Empathy out of the box. However, to make it work for VOIP, you need to install additional packages. Hence, do this in Terminal:
sudo apt-get install telepathy-sofiasip
Afterward, open Empathy and make your SIP account. Use VOIP call through Chat>New Conversation.