The IMEI or MEID number for your mobile phone acts as a unique identifier for that device. No two devices will have the same IMEI or MEID, which makes it a very useful tool for tracking lost or stolen cell phones. You can quickly retrieve your phone's IMEI or MEID number in a variety of different ways, depending on your device. Open the Phone app. You can retrieve the IMEI/MEID number on virtually any phone by dialing in the universal code. This code is entered using your phone's Phone app or physical phone buttons.Whenever the phone is switched on after putting a SIM, it tries to register on network and phone's IMEI number is sent to service provider during this process. So service provider will come to know about the latest number inserted in phone and they can then track the phone. If there is no SIM in phone or phone is kept switched off, it cannot be traced with IMEI only.