Account was shut off and still getting billed

My account was turned off, I paid the final bill and turned in the modem. A month later I get a notice saying the account is past due. When I log in, it shows the accound is connected and active with my bill past due. Late charges and everything. I try to contact customer service and get the automated system that runs you in circles for like 20 minutes before saying the office for billing is closed. So much for that 24/7 customer service as advertised.

Maybe if I ignore the bill, they will contact me and I can deal with it at that time. 

I've nevver seen customer service like this. No way to actually contact someone.