how can I get my IMEI from my old IPHONE 8 Plus that is no longer in my devices ?

I Switched cell phone plans and they had a promotion to send in the old phone and they will apply a credit, they lost my phone, it says it was delivered but since I set up my new phone that phone is no longer stored in my apple devices and they need the IMEI number to find it in there system and I did not write this down and I am smacking myself for it.