I lost my phone and can't find the IMEI number because my phone is gone and don't have the original box that the phone came in, how else can I find this number?

Here is Apple’s support article—>Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

If you don't have your device

There are several places to check that might show the serial or IMEI/MEID number.

1. Go to appleid.apple.com in a web browser on your computer.
2. Sign in with the Apple ID that you use on your device. 
3. Scroll down to the Devices section. To see the serial and IMEI/MEID number, select the device.

If you still need help, here's what to try next:

• Do you have a different device with iOS 10.3 or later that's signed in to your Apple ID? On that device, go to Settings > [Your name]. Scroll down to see any devices signed in with your Apple ID. To view the serial and IMEI/MEID number, tap the device name.

Axel F.