Originally Posted by jibi
EDGE was just an enhancement over GPRS. It doesn't allow that, either. UMTS does, though. But think of that as a completely separate technology that aligns with GSM evolution but completely different hardware. You can receive TEXT/SMS messages while on a phone call, but you cannot read MMS content (can receive) or emails on a BlackBerry (MMS is facilitated through data for the content).
I think you got this bit slightly backwards. You can READ any MMS that is already downloaded to your device while on a GSM phone call (regardless of the top of device you have). But you cannot receive a NEW MMS on a current call. This is why MMS has a maximum network pending time (pend to send to device) of 72hrs. ;) This is for GSM/GPRS/EDGE.
However, apparently many of Nokia's high end devices claim a certain Class 32 EDGE with ability to be able to do voice and data on EDGE simultaneously (N95, N82, etc); I havent' seen anyone prove this or tested this myself though.