Can you use a Cingular Sim card in a t-mobile prepaid phone?

On some t-mobile phones you can use your cingular/at&t sims card,but on some phones after putting a t-mobile sims card in a cingular phone may cause the phone to lock up.....BUT u can unlock your phone by calling your network from another phone or logging on to your account on the internet(I PERFER CALLING YOUR NETWORK).You can also unlock it but putting another cingular sims card back in a cingluar phone then your phone should work correctly if still locked call your service provider but your t-mobile phone shouldn't get locked if u put a cingular sims card in it your phone should work perfectly fine but if not call u service provider.