I called the billing office and was told they took the collection back (or whatever the correct term is) back when I made the payment. I let her know that they're still harassing me and even demanded my bank details, she's going to contact them and again tell them the account is no longer in collections.

I get those calls all the time. I first got them because someone with my father's name was racking up bad debt in my home state and they called me. (he got those calls too.) Then I got them because someone with MY name was racking up delinquencies. (my dad got those calls too.) Now I get them because one of my house seller's kids apparently has defaulted on something. I actually think I might call my seller to let her know, since her kid is just barely out of her teens and this is the kind of thing you want to nip in the bud.