unlock iPhone

I got an iPhone xsmax  lock,my friend gave it to me in my birthday.I want to unlock this phone but I had a problem
I checked iPhone imei on att.com, results that my iphone has previously been reported as lost/stolen device by an AT&T customer. but I think it just a confused..Immediately,I talked to my friend about this phone,he provided some information in following:

He ‘s livings in US and he has used this iphone for a long time.One a time,he left it in his school and he didn’t find it so he reported to AT&T for his lost immediately

He was lucky to find it in after one month.The cleaner in his school found the phone and keep it for him but she didn’t know how to use it or connect to him because my friend has returned to his country at this time

And when he is in US,he is reported that his phone didn’t lose,the cleaner gave back for him.He was very happy but he forgot to tell at&t that his phone is got back.
I’m sure that my friend tell me the true . I will very grateful if you can unlock my iphone

Thank you for your patience and hope you will consider my request.