SIM cards are approximately the size of a postage stamp, and very easy to lose when they are out of the phone. The SIM card stores the phone's number, so if you have lost your T-Mobile SIM card, the phone will no longer have the number. If you can't remember the phone number, there are a few ways to get the number from T-Mobile. Once you have your phone number and a new SIM card, you can activate the SIM on your account.
Check your bill if you have a postpaid account. The phone number is printed on the bill.
Contact T-Mobile customer care. A customer care agent can look up your phone number by your name or social security number. You can call customer care by dialling 611 from a T-Mobile phone, or calling (800) 866-2453 from another phone.
Go to your local T-Mobile dealer. You can find the address in the phone book, or by using the Store Locator page on the T-Mobile web site. An employee can look up the number and sell you a new SIM card.
You will need to purchase a new T-Mobile SIM card to use T-Mobile service. GSM phones will not work without a SIM. You can buy SIM cards at T-Mobile dealers, or online.