I am using a factory unlocked 3gs (32GB) which since being upgraded to ios4.1 and now the latest 4.2.1 has suffered from calls always terminating after 2-3 minutes into a call. This never happened using 3.1.3 and other family phones on 3.1.3 do not have this problem. Is it possible to jailbreak the 4.2.1 firmware and downgrade it to 3.1.3. I appreciate a jailbreak for 4.2.1 may not yet be available, but when it is will it be possible to downgrade it? Otherwise any other Ideas other than going into an Apple store here in the UK. The phone is out of warranty.