Billing after cancellation

This in reference to Long Distance Service at a business.  There is no category/topic choice below for this service.  Hoping this gets to someone who can help. 

I canceled our companies long distance service on 1/27/23 via phone and paid our final bill then.  Was a long and grueling process of being on hold and transferred to various agents.  I then receive another bill and was forced to contact yet again via phone on 2/7/23 and again another long and grueling process.  Reps can't find info regarding my previous cancellation despite giving them the confirmation/reference code I was given.  Finally, after almost hour of being on the phone I was assured that our account was closed, at least that's what I was told and given another confirmation/reference code.  Why can't someone call in and express their need then be allowed to hang up while your reps finish their business then call us back or email with a confirmation that the customer request was completed instead of keeping us on the line?  WE ARE THE CUSTOMER...what happened to customer service?

Well guess what?  We our still receiving invoices which I refuse to waste company time on, yet again, to deal with this hoping AT&T would recognize the mistake and correct it.  No such luck.  We now receive a "Final Bill Notice"...really!!!  Is there nothing in our account (closed account) that would prevent such billing to go out?  To boot, we have been Spectrum customers for years so there is no reason to continue to send us anything.

Who are these agents/representatives that I am speaking with and what are they being taught?  Upon speaking with associates, clients and friends, a lot of them have experienced this same treatment.  Continued billing despite canceling/closing their accounts.