Originally Posted by typical

It could be the iPhone is only a GPRS Class C device - but probably more likely is that the AT&T network is a NOM 3 network. In other words, it doesn't matter what phone you have - when you're attached to GPRS, the network won't be able to route calls to the phone.

Edit: Having said that, some people over on Howard's are saying Apple told them this is a design decision by Apple - the network will route the call but the iPhone will silently refuse it. Pretty surprising if so, though it neatly avoids some nasty audio routing race conditions