Deleted my old phone from my iCloud account, reset the phone, then found out the phone wasn’t unlocked. How do I find out my IMEI?

I put in the new sim before I reset the phone and it was working so I thought the phone was already unlocked but I found out it wasn’t when I tried to set up the phone. I need to find out my IMEI number so I can unlock the phone but I can’t get into the old phone. Could I take it back to at&t where I got it and have them unlock it in the store since I financed it with them and had it on my plan? Or is there something I could do myself?