AT&T will flag old debt for 7 years and attempt to collect during that time. If it's older than 7 years they will not collect, they will simply run your credit.

So yes, it is possible to go back to AT&T after 9 years. You will not be rejected, but you may get a deposit.

At the same token, let's pretend it was less than 7 years old, they would simply ask you to pay the debt and run your credit.They do not hold grudges over old debt less than 7 years, as long as you pay it off.