Harassment from AT&T employee

I wasn't sure where else to post this so I'm posting it here. There is an employee with AT&T in the Upstate SC area who is a former associate of mine in the entertainment industry who spent the better part of a week at the end of June harassing me after his services with us were terminated, and after a month of not hearing anything more insults and rants, thinking he had finally let things go, I receive a nasty little insulting message again from what appears to be one of his many "burner" phones, and I want it to stop. I can provide his "main" mobile number, his name, his address, and at least one of his burner phone numbers. Attached is a screenshot of the confusing message I got out of the blue, but I know this message is from this same individual, and I'm wondering if he needs help in addition to discipline, but I am tired of the "Cape Fear" type nonsense.

The messages are borderline incoherent (really unsure of even what he means with some of this stuff), they're nasty and it's not very good of an image for one of your engineers to be presenting. If you can just tell me who I need to report the problem to there at AT&T, or if I should just go straight to the authorities, I would appreciate the help.

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