When it drops calls, are you losing signal completely? I ask because I've had issues with my phone dropping off the network entirely (i.e. the signal icon has the little cross through it). Sent my phone in for repairs and apparently they replaced the ribbon cable to the antenna which for the moments seems to have solved the issue.
I am in South Africa, on the Vodacom network but I did a few tests and it wasn't 1) the Service Provider (swopped SIMs with someone on another network) 2) The SIM 3) The software (tried a Hard reset) so my guess was the phone was faulty and it does seem that this was the case.