To register or update your T-Mobile SIM card, log in to the SIM page of the T-Mobile website, enter your new SIM card number, and follow the instructions on the page. This process takes just a few minutes.Continue Reading
Find your new SIM card number on the bottom of the rectangular card that holds your T-Mobile SIM card. The SIM number begins with 8901.
Visit the T-Mobile website SIM card activation page. As of January 2015, the page URL is http://www-t-mobile.com/SIM. Log in using your My T-Mobile password and mobile number.
Follow the steps outlined on the page, and enter your new SIM number as requested. It may take up to 2 hours for the registered SIM card to work.