Unless you get a lot of calls the chances of an incoming call being blocked by 10 minute polling is quite low I would say.
The theory is good, but perhaps there's more going on over GPRS than just email?
Any activity over GPRS will block the call, so perhaps you have an IM client running, or frequent web page refreshes, or something like that...?
I'm assuming your don't have Flight Mode on, and that the device doesn't actually "ring" (even if it does it silently)
Before you go too mad check the Sounds and Notifications under Settings - in there you can tell it what to do when a call comes in, vibrate, show a message, ring etc (sorry - you may already know that!)