As far as I understand, the only way to place a call from an app is using the tel:// URL scheme, which launches the native phone app. But the Truphone app available on the App Store can place VOIP call even when there is no WIFI available. They say they do it by making a GSM call to their server and routing the VOIP packets over the internet from there.
Rerference: http://blog.truphone.com/2008/12/truphone-on-the-iphone-now-works-without-wi-fi.html
How can I do something similar in my app?