lost phone found

Six months ago I lost my samsung note 8 phone.  I have been an att customer for 15 years.  I needed a phone in my business, so I made an insurance claim on my account and in 24 hours received a new replacement phone.   A couple of months later I found my note 8.   A few days ago I tried using my "lost " note 8 and learned that it has been blocked for use on the ATT network.  It is a good phone and I could certainly use it in my business as a backup phone.  How might I go about getting it unblocked from the ATT network.