I have now been to the apple shop so many times, that this time i lost it!
I have had a couple of solutions, the sim worked for me, but then it started happening again.
So I said to the guy at apple, that the problem started happening when my phone was in warrenty, which i am not sure that it did, but it was worth a trie. For the first 30minutes he wanted me to pay £140 for a new iphone, but after lots of complaining, he gave me a replacment one for £55!!! so if ur problem started wall in warenty, I think u have a chance of getting a new one cheaply, worth a trie, worked for me, all my problems are gone, i think my phone was a couple of years old, and I still got it!
good luck! 🙂