For the longest time it was taboo to even talk about VoIP on the iPhone, because Apple simply would not permit it. As luck would have it, there is now a legitimate and free way for you to conduct such activities if you deem necessary, and it can all be done with an app called fring.
fring isnt new to the mobile VoIP industry. In fact, its been around longer than the iPhone itself. And now, iPhone users can exploit its features fully, with the expense of just a few minutes of downloading their app from the iPhone App Store.also give you access to msn messenger, twitter, google, yahoo, SIP Clients and AIM