Unaccounted for debt

My mother gets calls from RCS every single day for an outstanding balance. My mom's a pensioner so I assist in helping her with payments and billing. So I've explained this to Jet or RCS whoever they are they just run through their greeting and start demanding money. I made sure to pay the balance in full in November. I even sent proof of payment to *** as per the agents guidance. Now I will not easily escalate someone that's why I don't bother asking their names but the consultant the other day was extremely rude saying that "you shouldn't be involved" after I've explained the situation. So apparently there's different departments and collections don't know what is sent to payments. Are they on different planets? Can they not forward it or does nobody care do they just want money. I also asked for a breakdown of this miraculous 2k when the balance was paid in full based on the statement sent in November. At this point I don't think my mom should be dealing with their harassment, multiple calls when NOBODY is willing to meet her halfway. I need regional head office contact details or I'll start recording calls, blocking numbers and my mom won't have to answer any calls. I want to get this sorted but RCS and Jet does not meet me halfway. Get it together!