You don't need to press the call button; your iPhone's IMEI number will instantly appear on the screen. Alternatively, you can find the IMEI number by opening the Settings app and going to General ...

This application was developed from the earlier applications, Find My iPhone and Find My Friends ... and support which 3rd party apps can not. IMEI Tracking: Knowing your phone's International ...

There are two ways to find the IMEI number on your iPhone. 1. Start the Settings app. 2. Tap "General," and then tap "About." 3. Scroll to the bottom of the page, and you'll see the IMEI number.