AT&T and T-Mobile both use the Global System for Mobile Communication, or GSM, network. As a result, it's easy to switch between the two carriers and keep your existing phone and phone number. So easy, in fact, that AT&T attempted to acquire T-Mobile and absorb it into its network. However, as of this publication, T-Mobile still provides a viable alternative to AT&T customers interested in changing cellular service providers.
Determine if your phone number is eligible for switching. T-Mobile provides a tool to assess whether your phone can switch to T-Mobile (full link in Resources). Enter your phone number and click "Check Eligibility."
Select your plan on the T-Mobile website (full link in Resources) or by shopping at a retail outlet. You can also call their customer care at 1-800-866-2453.
Provide your existing account information to T-Mobile and authorize the transfer of service. Do not cancel your existing service with AT&T as it can take up to two days to complete the transfer. T-Mobile contacts AT&T to manage the transfer of service. You receive a text message confirming the transfer from T-Mobile when the process is complete.