I was using my iPhone 3gs as a prepaid T-Mobile phone but got tired of having a jailbroken phone and of the bad service. I bought a cheap AT&T GoPhone and activated it. I then transferred my number from t-mobile to the AT&T GoPhone. I went to plug the sim card into the iPhone but it comes up as No Service. When I go to settings and click carrier it recognizes the AT&T network but when I try to connect to it it says network lost. I really need this phone for work and can't afford an AT&T iPhone contract. How do I get the iPhone to recognize the sim card?