There used to be a problem on very old phones where if your using a 2G network (which GPRS is) and you have an active data stream calls would be diverted, this was because the system only allows either a call or data. In newer phones (X2 included) the phone will hold the data stream when you make or recieve a call. You shouldn't have any problems with missed calls or not being able to recieve them while a data call is active. Could be a faulty handset, or you could try re-installing the latest firmware, though I can't see that being the issue. Either that or your network has a problem.
