High interest rate

I have used Telkom smartboard data for the past 3 years now and never had a problem with it in terms of payments but as of late they lack to debit their money on the agreement date. On the month of June 2023 they tried to debit from my account but the payment was rejected of which I was in arrears for that month but on the 1st of July 2023 the money was deducted from my account. When I received a statement for it showed my account was still in arrears of which I understood because the late payment wasn't showing. But what I need to understand why is their arrears charge 100 percent interest? From my understanding there is a certain rate change for arrears in South Africa. Telkom was not my account which miss payment. End of July 2023 I had to pay double plus the current month. On August 2023 I wasn't expecting to pay because I paid 1st first of July and end of July 2023 the money was already reflating on the statement. So I want to know what happened to that payment and why they charge 100% interest? During the first week of September 2023 I tried to contact them but they make answer their robot online and promised to call you back. And never get back, I am very much disappointed on Telkom.