Hi Steph. Try these following methods:
1. Enter *#06# on your ZTE phone's key pad. When the final # is entered, a 15-digit number with the title "IMEI number" will appear.
2. Open the battery cover on your mobile phone and remove the battery. The IMEI number should be located on the white label under the battery (see image below). It will be 15 to 17 digits long with no letters or slashes. Only the first 15 numbers are required. source