We've been beta testing a promising new Web-based app for the iPhone, dubbed RingFree (go to https://rf.com on your iPhone to try it) that offers the ability to make calls through a variety of VoIP SIP providers using normal voice calls on the AT&T GSM network, or any other standard iPhone voice provider. The method is similar to the one used by Talkety, where calls are routed through an external server instead of through the iPhone's data connection, but it allows the use of any designated VoIP provider or your own Asterisk/SIP server and doesn't require you to receive a call in order to make one (like Talkety).
Here, in a nutshell, is how the app works: A user registers with RingFree, entering his iPhone number and providing some other information. The user is then prompted to call a country-local number to confirm their information by entering a PIN. Once logged into the site, the user selects from a list of pre-defined VoIP providers (including VoicePulse, Gizmo Project, PhoneGnome and others) or defines his own by entering a proxy address, username and password.
The net effect is this: you can (for example) use your local, US AT&T minutes to make calls to the United Kingdom for only the cost of the local minutes plus whatever your VoIP provider charges. VoicePulse, for instance, charges just over a penny per minute for calls to the UK.
We've been using a test SIP server to make calls, and the results are impressive. Voice quality is relatively good, the calls go through fairly quickly, and we didn't need to be in range of a WiFi network nor be in particularly strong EDGE coverage for the service to work properly.
RingFree also lets you make calls to Google Talk and MSN voice accounts.