For Google™ Voice users, Sipdroid can now create a new, free PBXes account that is automatically linked to an existing Google™ Voice account. The new feature requires Android 2.0, or above, and Google’s app connected to your Voice account.

via sipdroid – Free SIP/VoIP client for Android – Google Project Hosting.

Just found this site and sipdroid looks like an interesting VOIP solution for a tablet wifi.  Will download to see how it works.