The IMEI number stands for International Mobile Station Equipment Identity, which is unique to each and every mobile. It consists of 15 digits. As this is a unique number that helps a lot in case of a loss of your phone. This IMEI number is very important to the GSM network rather than any other one. They use to check the validity of the mobile device and in case of the phone has been stolen the provider can stop accessing the network. In short they black list the phone so that no one can use the phone at any network. This IMEI number is only used identify the device there is no any special functionalities occupied with this.