Read all about Check IMEI.
What is an IMEI number?
Mobile devices are one of the most commonly stolen items. T-Mobile, in cooperation with government agencies and other national and international carriers have joined together to help eliminate the use of stolen devices. Your International Mobile Equipment Identity number (IMEI) is a major part of this plan. An IMEI is your 15 digit serial number which identifies your wireless device.
Why should I verify my IMEI number?
If a device is reported stolen, the device will be unusable on most carrier networks (including T-Mobile,) even if the SIM card is changed. Verify your IMEI number to ensure your device will work.