iPhone 4 drops calls constantly despite full signal then freezes

I'm at the end of my tether!  My iPhone4 drops calls for absolutely no reason at all, at least 50% of the time.  It doesn't matter how strong my signal is, or whether the call is outgoing or incoming.  The person on the other end doesn't sound like they are breaking up either - it just goes suddenly silent.

Then, when I press 'end call' the phone freezes on 'call ending' - the only way out of this is then to reboot the phone.

I've been back to the Apple Store in Glasgow twice, and to their credit they have replaced my handset twice within the last year; however despite my THIRD handset doing exactly the same, they have told me that no more replacements will be given.

I have tried doing a master reset and setting up again as a new phone to eliminate the possibility that something within my back-up is corrupt, but the end result is the same.

I have five different Apple devices at home and am a huge fan, but I'm afraid this is making me envious of my friends who have Android phones (sorry, didn't mean to swear!)

Any suggestions????

Thanks