Refuse to refund for cancelled service due to incorrect billing

I cancelled my ADSL and landline service in on 9 November 2020 by giving the requested 30 notice. All my services were cancelled on 9 December 2020 including email account, website logins, ADSL line and landline. However I received a bill for 9 December 2020 to 8 January 2021 for the full amount, while I had no service. I was handed over to lawyers for non payment. I paid the outstanding bill, which I disputed as I followed all the requirements for cancellation of service. Telkom reversed the amount for that bill and I now have a credit on my account. I requested a refund, but it was denied cause I was handed over. Clearly I now have to pay for Telkom's mistake of incorrectly billing me and then wrongly handed me over to lawyers. I don't understand why I have to pay for their incorrect billing. I just want my money back since I had no service to use, but still I must pay. It affected my credit score severely, but it was TELKOM's mistake. After years of paying my bills on time, never defaulting this is how I get treated, its blatant theft!