hi recently i got my hands on several iphone 3g phone and they seem to have a problem that " no sim" comes up from time to time. i have used 2 tmobile sim cards and one had the problem other pretty much did not. i think i found the problem to be little sim card movement in it's seating place as you insert it into the phone. 1 sim card has a little plastic left over from original pacaging so it fits into iphones sim card space without moving, but 2nd sim does not have that and moves slightly and seems to be causing the problem. i guess it could occur when you shake the phone enough to move the sim card. hope this observation helps some one.