iPhone says no SIM/no service/SIM failure

From the day i purchased my iPhone 3GS, the messages no SIM/no service or SIM failure have appeared meaning i cannot text, call or acces the net. I made an appointment at the genius bar and they said that it was the phone so after going to the store i purchased the phone from and to the apple store 6 times i finally got a new phone. However, the same message appeared so i requested a replacement SIM card from O2 but it hasn't worked at all and appears with the message no service.

Any help on this issue would be greatly appreciated,
esarah