T-Mobile provides users with an IEMI status check tool, which confirms whether or not a particular device works on the mobile carrier's network. Visit the IEMI status check page, enter the IEMI number on the device and click Submit to confirm network compatibility.
The IEMI, or International Mobile Equipment Identity, number helps prevent theft by giving each device a unique 14 to 16 digit serial number that cannot be altered. This number allows T-Mobile to shut down usage of a stolen device operating on its network, in conjunction with government agencies. If the device in question comes up as stolen, even with a new SIM card, it won't work on the T-Mobile network.