Well... Try this patch of magnificient fring. You should uninstall previously installed fring, download the file, click twice, iTunes will get it and during the Sync will install it to iPhone as usual. This should allow for all around the world 3G community to make and receive voice calls via any type of internet connection: WiFi, EDGE, 3G.
PS I know, it's impudent (a little bit to post a link to patch on developers board, but, I think they deserve it (and WANT it) because of suckin apple sdk rules. Dev team must obey the rules... and I am not. Enjoy!
Understandably, albeit unfortunately, the link has been removed off the forum; however, I am sure its out in the wild somewhere...