In case you are willing to port your phone service from T-Mobile to AT&T, but you do not want to change your present T-Mobile phone, then you can be lucky enough to retain your number. You can use your AT&T Sim card in your T-Mobile phone. Many of the GSM mobile phones, sold by the T-Mobile, are also compatible with AT&T, but only when they are unlocked for multiple carrier usage. You must only need to know that which mobile phones are compatible with AT&T and how they can be get unlocked for multiple carrier usage. Step 1. Find out the "Calling Frequencies" your T-Mobile Phone. The currently available frequencies are 1900mhz, 1800mhz, 900mhz and 850mhz. Most of the T-Mobile phones operate on the frequency of 1900mhz. However, some of their phones also offer two or three and even all 4 of these frequencies. To operate through AT&T, your device needs to work on 850Mhz AT&T frequency. You may find the available calling frequencies for y