Retrieving IMEI number for a traded-in iPhone 15 Pro

Hi guys, bit of a weird one, I had an iphone 15 pro which I have now traded in but where I sent it too are having an issue identifying it somehow, they have asked if I know my imei number, which I don’t. I sent it back in the box it came in which will have the number on the box, I now have an iPhone air with the e sim, is there any way of finding out what that imei number could’ve been as that phone is no longer on my devices section under my Apple ID.

[Re-Titled by Moderator]

Original Title: imei identifying