for AT&T, T-Mobile(GSM Service Providers) if you want to switch the SIM cards the phone must be unlocked. for that you have to purchase an unlock code...

You need to unlock both phones if you want to interchange SIM cards.

I have both an AT&T and a T-mobile pre-pay phone. I've had the T-mobile phone for years, and the AT&T Go Phone is new. I tried to swap the cards but both phones reject the other cards. The AT&T phone asks for an UNLOCK CODE (10 trys and you're out!). The T-mobile phone just reads WRONG CARD.

Right now, I want to use the T-mobile sim card (which works fine in the old T-mobile phone) in the AT&T phone.

Eventually, I want to to buy a specific phone and use the T-mobile sim card in it also. (No new contracts!)

Thank you for all good advice!