A:Answer Hi, as far as I know, Sprint phones aren't compatible with ATT networks. If your phone is GSM compatible (some unlocked phones are compatible with GSM and CDMA), then it will work on ATT, but if it is CDMA only, then you will not be able to use it on ATT.
Regarding changing services, if you move from Sprint to a different provider, then your phone will need to be unlocked by Sprint (unless you purchased an unlocked phone in the beginning). I think you would need to call Sprint to get your phone unlocked.
Regarding using a new sim on a phone that hasn't been used in a while--that is perfectly fine; however, when I wanted to activate my ATT prepaid plan with a new number, even though I bought the sim kit through Best Buy, I had to visit ATT directly since Best Buy couldn't add a new prepaid line. On the other hand, if you have an active phone number, I believe you can transfer that number to a new sim card yourself online.
