Unprofessiona Debt Collectors

I had a debt with MTN which was pulling from the other account, when I change the account number I was told at MTN store that the debit order cannot be paid, I have to pay cash, whereas, I am 120 KM aware from the MTN store. Now I am receiving annoying phone calls from unprofessional debt collectors, that have failed to give account where I can pay for the MTN.