Phone trouble

Hello. I updated to IOS 7 last week and my phone went back to its locked carier. Well, let me give you some background on the problem. I brought a phone, IPhone 4s from sprint last year. The bill got to be too much, So I had sprint turn my service off. I flashed my phone to T-Mobile, but wasn;t satisfied with them. I then asked Att if I could get a sim card from them and nuse my phone. they said yes. Now the problem comes in. I downloaded IOS 7 and now my phone is not working. It keeps saying that it is on the Sprint network, but I have an ATT sim card inside the phone. Does anyone know what happened? Do I have to unlock the phone to get it working again? Please help.