This is happening quite frequently.

Make a call, as it is dialling the call ends. I never hear it ring on my side but it does ring once or twice on the other side.
It often takes up to 6 or 7 attempts to dial but once it does finally get through, everything is fine.
It has never dropped a call once properly connected and talking to someone.

The problem happens with the phone in my hand AND on speaker phone AND with hands-free in the car.

Doesn't seem to be a problem with the network as there are 3 different phones in the household on the same network (3 UK) and the others never have this problem and this happens in lots of different locations, not just one area.

The phone has been taken back to the shop and they replaced it but the problem persisted.
Went back to the shop and they replaced the sim card and again, the problem persists.

So clearly it is not a problem with the sim card, the network or the particular device.

So could it be a design fault in the S3? I haven't been able to find anything to suggest other people are having this problem.
So I can only think there is some setting somewhere that might have be affecting it but I can't find anything.
All motion settings are disabled but since it happens when the phone isn't being held it shouldn't be that anywhere.

Completely stumped.