Credit Balance on Closed Landline Account

Closed an account in 2017.  There was a credit balance when the account was closed.  The refund has never been issued and statements continue to be issued monthly showing the credit.  Tried calling about the account numerous times but they say the account is closed and there are no existing records...BUT I keep getting told that a refund will be automatically issued after 90 days.  NOT TRUE!!  It's been over a year!  I sent a written request for the refund per AT&T's instructions to: AT&T Global Imaging Center, P.O. Box 16795, Mesa, AZ 85211 but it was returned as address not known.  Is there another address?