Learn here, how to flash your T-Mobile to AT&T in some simple steps. If you are using the wireless service of T-Mobile at present but you are willing to change it to AT&T, then you are required to make your phone ready to accept the AT&T SIM card . Usually, this process is known as “unlocking” which is incorrectly termed as “flashing” sometimes. Generally, the word “flashing” is used for the procedure of reprogramming any CDMA phone in order to work with another CDMA network. For instance, in case of switching services from Verizon to Sprint. The GSM network uses the SIM cards while the CDMA network does not. So, you need to unlock your T-Mobile phone to use it with the AT&T network , since it was programmed to only accept the T-Mobile SIM cards before it was purchased by you. Step 1. You need to call AT&T in order to launch a fresh wireless account. Speak to the AT&T representative and tell him that you want to use your existing cell pho