Mobile forward to VOIP app riddle

[Guest phlangeface]

Is there an easy way to forward normal mobile calls to a voip app for when there isn't any 3G/Mobile signal, but there is WI-FI available?

I have switched from a blackberry to the Galaxy S. There were no issues with the blackberry because it uses UMA.

My tiny brain tells me that it must be possible in theory?

BTW. I only need to receive the calls. I Don't need to make calls using voip.

Edited July 26, 2010 by phlangeface