I always get that now I switched to the Iphone never got it before. I just blame the phone, didn't think it was anything to do with 3. but then I also get no sim card quite a lot as well.
This has happened to me on 3 network. I put it down the the mast. I am within line of sight some 200 yards from it. So, usually bending the bars. It went down for one day. Nothing to do with the phone.