Expanding GTalk2VoIP, a service that lets you transform Google Talk into a true VoIP client, Talkonaut is a Java client that can be installed on your mobile phone and gives you access to Google Talk.
As expected, you can use this service only for text messages. If you want to make voice calls, you can't do that for free. "Talkonaut utilizes callback technique to make voice calls. This means, when it is necessary, our system (GTalk2VoIP gateway) calls back to your mobile phone, then calls to your recipient, after that it merges two voice calls in one."
Talkonaut works with any Jabber account and looks better than MGTalk, another mobile client for Google Talk, which is open source.