Contact AT&T directly to see about that "final bill" - there have been reports in the press of complaints by people that AT&T messed up on property closing their accounts after the final payments have been made.
https://consumerist.com/2008/03/31/att-lets-y ... -paying-for-it/
AT&T Lets You Close Your Account, But Not Stop Paying For It
By consumerist.com March 31, 2008
https://consumerist.com/2010/03/03/why-wont-att-cancel-my-account/
Why Won't AT&T Cancel My Account?
By Chris Morran March 3, 2010
https://consumerist.com/2012/06/25/cancel-cra ... -for-it-anyway/
Cancel Crappy AT&T DSL Service, Get Billed For It Anyway
By Laura Northrup June 25, 2012
I'm surprised you didn't receive a final bill in the mail from AT&T. I would think you would have received at least a few before your account went to collections. You might want to contact AT&T, as William suggested.