If you got a "past due" bill from AT&T, I wonder why you didn't also get all the previous bills they sent out before the "past due" notice?
I would get online immediately and check his credit report with all three credit bureaus. You can do this once a year for free. You may be surprised what you find there. Did your son have phone service in his name somewhere else at the time this service was supposedly hooked up? I ask because I was able to get phone service for my son in the mobile home he was living in without signing anything or paying another deposit ... I arranged it all on the phone! It was a separate number, in a different town, but in my name on their records because I already had phone service with the same company in my name, and I was ultimately responsible for the bills even though they were mailed to my sons address. It was just considered to be a second service in my name.
This same thing happened to my son about two years ago, and it's not as easy to sort out as they claim it is! In his case it was fairly easy to figure out who had been using his SS number ... when you enter my sons' SS number on one of the credit bureaus, HIS FATHERS' name comes up! What a mess! Some info is his, some is his fathers! HIS employer info, his FATHERS' mailing address, some accounts my son had previously, and lots and lots of unpaid doctor and hospital bills that were obviously not his, they were his fathers! The Social Security office said it is NOT a mistake in their system, good ol' dad HAD to have given sons' number instead of his own, and probably just used his first initial instead of first name, since they have the same initials!
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I'd get this settled as soon as you can, and also check those credit reports ASAP because if they did it once, Lord only knows what else they might have done! Good luck!