Account in errors

I have closed my Mr Price account in July 2018, in September 2018 i get a call from their call centre to sell me a sim card and airtime, i clearly told the call centre agent that i do not want any airtime or sim, the agent then said that it will not cost me anything to get the sim and that i can go to a MR Price branch and take the airtime if i wanted, i clearly told the agent i will not take it if i will be paying for it, because i already have a contract, she said that i wont be paying for anything. thats when i said okay.

MR Price then billed me for the sim card and the Airtime. i went to the closest Mr price and quarried this and asked them to go back and get a hold of the recording. they said they will get back to me.

After a few months Mr Price then reverses the charge. and now they want me to pay the interest on the amount. this is so unprofessional for such a huge company to not check that the account was closed and that they are billing stuff on a closed account and expecting me to pay for it when now the interest is more then what the sim and airtime would have costed, or they just changed the sim and airtime charge to a interest charge.

i have emailed Mr Price, i have spoken to their consultants, yet nothing has been done, and i still receive threats from their debt collectors .