IMEI, otherwise known as International Mobile Equipment Identification, is a series of numbers that identifies key elements of the phone that it can be registered to a single user and reported stolen if necessary.
The information that the IMEI is able to store on the phone is amazing. Not only is each number different, but each number can explain the origin of the phone, the model, and the serial number. This can come in handy if the phone is stolen or if the phone needs to be used as evidence for any legal action.
The original purpose of this IMEI number was so that if the phone were to get stolen, the phone owner could call the network provider and have the phone blocked so that it could not be used with any provider.
This purpose has been expanded to cover phone identification for legal processes.
In addition to the IMEI being located behind or under the battery, one can locate the number on the phone.
For those new phones, one simply has to dial the phone and wait for it to answer.