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?
As far as the phone's cell hardware is concerned this isn't a call. Its just a 2 way data stream. They are activating the mic and encoding its input and sending that stream to their server, while they receive another stream and play it on the speaker. Their servers then decode the streams and then connect it to a normal phone system so you can talk to someone who doesn't use their service.
Do you want to allow users to contact people outside your userbase? If so that's going to require you to set up a server to decode your streams and then find a VOIP provider that has an API so your server can stream to them. This is possible but it costs.
If you only want user to user this can be done for free but its almost identical to the previous option, just take out the VOIP provider.