Unfair demand

I had set up a debit order for RCS. They debited as per instruction and I never missed a debit. After some months the debit stopped. I took it that am paid off since I didn't cancel the debit order. About 14months later I got a call from RCS saying my account is in arrears I must immediately settle the amount. At first a lady by the name Amanda called me and she was very demanding. I told her that I didn't stop the debit and the company didn't notify me of any changes or send me alternative payment details. Why would they contact me more than a year after they changed things? She kept on calling me until I told her that she is harassing me for something I am not responsible for. Then a male called me with the same story I told him the same thing as well.