Call Failed Issue iPhone

My iPhone4 has got a 'Call Failed' issue and doesn't allow any outgoing call. I'm still able to receive calls, send/receive text messages, use 3g and wifi - just not able to call out.  I contacted my Service Provider and they recommended me to reset my network settings which didn't help. I switched my SIM card with another service provider and still the problem exists. I also restored my iPhone but didn't do anything. Tried to update the iOS to 5.1.1 (current version is 5.1) but ran into an error (6009). Not sure what to do next. Anyone please help?

Thank you very much!