Suddenly my IPhone won't make or receive calls, but I can send and receive txt msg and use internet and iPhone 4 has full 3G signal?

Problem started a week ago. I can't make or receive calls but can make and receive txt msg and use internet.I am a Vodafone Ireland customer. I have been to the store 4 times this week. They have given me new sim and say the problem is iphone4 . I received the iPhone 4 new as a Christmas present. It is only 2 months old . I have downloaded latest IOS and reset connection settings. Please can you help me