POOR SERVICE & HARASSMENT

I had RCS account some years back, but then had to cancel same after I got medically boarded from work. I paid settlement amount and sent cancellation letter requesting. However, they have been harassing me with debt collection calls, claiming that I have to pay some fees. I don't understand why should I still be paying anything when I explicitly requested the account to be closed. It's been 4 years since I was medically boarded and canceled the account.