[CrackBerry Question]

Hello,
I've bought on Ebay, not working Q10, (issue with software), I managed to fix it and the phone is working already. I"ve found out that the phone is locked to ATT network. I've checked IMEI and the phone is ok, not reported lost or stolen, so the IMEI is clear. Unfortunately the sticker with all the info is missing, and I do not have original box. I was trying to unlock the phone everywhere, but every place I tried they ask for PRD number. IMEI is not enough. Is there any way to find out what the PRD number of that phone is? Should I go to ATT and ask if they could find the number? or there is other option. Thnx for any ideas