Viber is a free VoIP iPhone app that works over 3G, and WiFi of course. There is no registration: just enter your mobile phone number. You also get free SMS.
[…] and the sound quality is much better than a regular call.
Incredible claim, but I don’t doubt it since I’ve experienced many Skype calls that have better quality than regular calls.
Calls between Viber users are free so you’d want your friends to get on Viber, too. But there is a catch: it seems Viber sends a SMS invitation to all of your friends. That could get really expensive. Also VoIP uses data so keep an eye on your data usage.