SIPDROID (Android app) and Voxalot
 

I have been using Sipdroid on my early generation HTC Magic and has been working wonderfully and in someways perhaps better than sip client on Nokia N series. With Nokia, I noticed that connection timed out and would reconnect when it feels like. The benefit , in this way is greater battery life at the cost of receiving incoming calls.
Sipdroid has been less power hungry provided you use TCP rather than UDP.
And it works with WLAN, 3G and Edge data connection.
It seems to be more stable than Bria for iPhone. Bria by Counterpath is not a free application but Sipdroid for Android is.
Please feel free if anyone need any more on this.