iPhone 3G Fails to Make/Recieve calls? Everything else works fine

Hi All,

Yesterday my iPhone 3G started acting up, I get a call failed message when trying to place a call and incoming calls cannot be answered. Every other feature works as usual including texting, voicemail, and internet access. I didn't change any settings and I have in no way hacked my phone.

I've tried turning it off/on numerous times to refresh the connection, I've even tried to restore it and it didn't work. What else can I do?

Please help! What has happened to the PHONE in my iPhone!!!!