My husband bought a iphone3gs. We put the tmobile sim card in, set it up as a new phone, it worked fine for a week or so. Today I woke up and noticed it said NO SERVICE. Thinking if I restore it, it might fix the problem. Well now when I get to the activation screen it will get to the screen where it says "insert valid sim card. On my itunes page it says the sim card inserted in this iphone does not appear to be supported.. Ok first off how did it work on tmobile for a week or so and all of a sudden it is not compatible. It is AT&T phone. It had to be unlocked to even work for that short time.. I thought once a iphone is unlocked it is unlocked forever? I have tried to restore it 5 or so times and I get the same thing. HELP ME PLEASE