If you're changing service from T-Mobile to AT&T and you love your current T-Mobile phone, you may be in luck. Many GSM phones sold by T-Mobile are compatible with AT&T when "unlocked" for multiple carrier use. You simply need to know which phones are compatible and how to unlock them for multiple carrier use.
Determine the "Calling Frequencies" for your T-Mobile Phone. The available frequencies are currently 850mhz, 900mhz, 1800mhz, and 1900mhz. T-Mobile operates on the 1900mhz frequency, but many of their phones offer 2, 3 and even all four of these frequencies. In order to operate via AT&T your device will require the 850Mhz AT&T frequency. You can find your devices available calling frequencies in the phone's user manual or typically on the original retail box. If you do not have your manual or box, GSMArena is the authoritative voice for GSM specifications.
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Determine how long you have been a T-Mobile customer. If your contract is at least 121 days, T-Mobile will typically provide you with your "unlock code" for free. If you have not had your phone on your current contract for at least 121 days, T-Mobile will not provide the unlock code. Simply call T-Mobile customer service at 1-800-Tmobile and request your device's unlock code. Type in the code exactly as provided to you by the customer service representative. After entering the code, reset your device if it doesn't do so by itself and then insert your AT&T Sim card--you should now receive a signal.
The unlock code will usually involve typing a set of numbers with several of the device's other keys included in the process. For example, holding down the two soft keys on your phone while typing "#1293_" and then a code such as "_1234." The process can widely greatly based on your device's requirements.
Purchase the unlock code for your device online if T-Mobile will not provide it or if you did not buy the locked T-Mobile phone from the provider. GSMLiberty is a trusted unlock source for most cell phones. You will need to provide the site with your "IMEI" number, which can be found under your phone's battery. Enter the unlock code just as provided to make your phone ready for AT&T. Insert your AT&T SIM card and test the phone.
Setup your multimedia messaging service options via your phone's menus if you plan on using multimedia messaging on your device. Each carrier's MMS settings vary based on the phone you are using. You can find MMS settings on popular cell phone sites such as HowardForums and on the AT&T Wireless webpage.