How do i unlock device apple

Hello At&t . I am not a customer of at&t company. I didn't find support service email on the company's website. That's why I write to you here. Please help me. One person suggests to me to buy mobile phone apple iphone 7plus 128gb with IMEI XXXXXXX and serial number XXXXXXXX blocked (sim lock) by at&t company. APPLE Icloud not blocked. When I use the unlock service (https://www.att.com/deviceunlock/?#/unlockstep1), it's notify me that this mobile phone was lost ot stolen. PLEASE answer me is it true? was this phone really lost or stolen? please give me more information about this device. is it possible to unlock this device? and how can i do it?
Thank you to find time to handling my request. If I can provide you with any further information, please let me know.