Siax
Version: 1.03
Siax is a perfect VOIP application for your Iphone. With Siax you can add any or multiple VOIP providers to make cheap calls when you are in a WIFI Hotspot or you have a 3G/UMTS connection. Siax supports SIP and IAX2 protocol.
Siax is a VoIP client for the iPhone or iPod touch which works over wifi, EDGE, or 3G. It has support for mutilple SIP and IAX2 protocal accounts, and has DMTF support. Siax has a simple dial pad where you can choose to either call through IAX2, Cell provider, or SIP on the same page, and you will have your contacts already synced to your phone avaliable and give you a recent calls page.
In my testing calls sounded pretty good on wifi, over edge (when I got it to connect) there seemed to be some delay but sound quality was about the same, I wasn’t able to test 3G without a 3G iPhone. Of course your mileage may vary, especially on edge as AT&T isn’t the best out here.
Perhaps if you’re near wifi most of the time, Siax could be good for you, but over the cellular network I did not have as good of luck. But your cell coverage and speed could be the difference if this would be your new phone application on the go.
[Source - BigBoss] Cydia