Can't Recieve Text Messages

Question

I have a Sony Ericsson W960i which won’t receive text messages. I can make calls and receive them and I can text other people they just cant text me back. The strange thing is when I restart the phone I get the messages I have had up to that point but then it goes back to a state of not being able to receive text again. I have tried the SIM in another phone and that seems to be ok and I have tried a different SIM in the W960i which seems to have the same problems so the problem seems to be with the phone. Is there some stupid setting with the phone? or should i be contacting o2?