ATT Wireless 2nd tier Tech support?

I know this is the wrong place to ask, but I have some users that are having some odd issues trying to a certain number(it’s a dr office at a hospital) on their cell phone when on AT&T.
I have had multiple people try the same number(including myself) and it does not work.
Same number works from a land line with no issues.
Someone from outside the area could call the number with no issues as well, so the problem might be somewhat localized.
Not sure if the issue is on the AT&T side or the recipient side.

Can someone point me in the right direction?