Presently Empathy supports voice calls through gtalk. The installation instructions are available here. Just for convenience, I am reproducing the set of instructions necessary to get you started with it. Visit the above site for user comments and more information if things don't work out for you.
Use Synaptic package manager, go to Settings->Repositories->Third Party Software and then click Add and add the following line
deb http://ppa.launchpad.net/telepathy/ubuntu hardy main
Then, Install the following packages using Synaptic Pagakage manager.
- Empathy
- telepathy-gabble
- telepathy-mission-control
- telepathy-stream-engine
After successful installation, open the Empathy application and use the menu, Edit->Accouts to add your gtalk account. Enjoy using voice chat with your favorite gtalk account using Empathy in Linux.