You will have a somewhat harder time working with the Virgin Mobile branded handsets in achieving free wifi calling. It might take a little bit of hacking on your part. Definitely consult @ howardforums.com in the VM sub-forum to see what options currently exist.

OTOH, you could quite easily configure an iPod touch to do this, and for free. Last night I finally got around to installing the Talkatone app on a 3rd Gen iPod Touch which enables free inbound/outbound VoIP calling. It uses my Google Voice account to be forwarded to the Talkatone VoIP app where I can send and receive (domestic)calls for free. There are some helpful instructions on getting the ball rolling at the Talkatone blog-

http://blog.talkatone.com/