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Before removing the battery, power down your phone using the Power button. This will prevent any data being lost and keep apps from being corrupted when the power source is removed. This method only works for phones with removable batteries. It is not possible with iPhones or other phones with affixed batteries. Gently remove the battery from the phone. Generally, you will need to push it slightly towards the bottom of the phone before pulling it out. The location of this will vary from phone to phone, but the IMEI/MEID number is usually printed on a sticker that is affixed to the phone underneath the battery.  If the phone has an IMEI number but you are on a network that uses MEID numbers, ignore the last digit (IMEI is 15 digits, MEID is 14 digits). GSM networks like AT&T and T-Mobile use IMEI numbers. CDMA networks such as Sprint, Verizon, and US Cellular use MEID numbers.
Power down your phone. Remove the back from your phone. Take out the battery. Find the IMEI/MEID number.