Your SIM card acts as the "key" to T-Mobile's network "lock," allowing you to make calls and send text messages. When SIM registration failures occur, your phone loses access to these features until the issue is resolved. This problem is sometimes correctable without replacing the SIM card, though purchasing a new one may be an unavoidable solution.
Reseating the SIM Card
Registration failures may occur if the SIM card has become loose in its socket. By removing the back cover and pulling out the battery, you can access the SIM card's slot; pull the chip out and gently insert it once again. Make sure that the SIM card does not wiggle around after reseating it. After putting the phone back together and powering it up, the issue may be resolved.
Cleaning the Contacts
Registration failures can also occur when the SIM card's contacts become dirty, as grime can prevent a solid connection with the phone. Dip a microfiber cloth or cotton swab in a little rubbing alcohol and gently clean the golden contacts on the SIM card's surface. Let the SIM card air dry before putting the phone back together and testing for resolution.
SIM Card Replacement
Should registration problems persist, you will need to purchase a replacement at a T-Mobile retail location. You can also call customer service and purchase a new SIM card over the phone, though it will take a few days to receive it by mail.