Living in Europe last 7 years and getting a collection request from AT&T U-Verse showing up on my credit report.

For the last 7 years I've been living in Europe, and yet there's a collection showing up on my credit report for an AT&T U-verse charge of $750.

When I called the collection agency they say it's for a charge that occurred with AT&T (and the timeframe was while I was already living in Spain). I left the US with my debts paid and impeccable credit with a credit score over 830. It's now dropped significantly just on these alleged collections/debt and I'm very frustrated and upset with AT&T.

How can I get this resolved?