Internet Telephony Provider Ring Free rf.com is another VoIP iPhone-specific PBX service. The service will allow you to make international calls and calls to online providers like Skype using your normal iPhone minutes. So if you have a friend who’s on Skype but has no Skype In access, you can call them just as if they had their own number.
» RingFree – Internet Calling for the iPhone

Sipgate is another VoIP client for your iPhone much like Fring but then again not quite. The difference between Sipgate is the fact that you can call any number directly from the app without have extra accounts. So to use this app, you won’t need a skype or any other subscription based service. You can use sipgate to place and receive calls over Wi-Fi using any SIP-based VoIP-service. And the best off all is the fact that you receive 111 minutes for free.
» Sipgate – Free native voip iphone client