Interest charges

I do not have a credit agreement with telkom. This month I paid my accounts at night and my telkom account was due on the 28th April. It was paid at 01h44 before opening of business on the 29th. I was charged R5.07 on a bill ****axed) of R389.23. This amounts to 476% per annum interest. I never agreed to this!!!!! I phoned the call centre to complain and spoke to Lizanne. I confirmed my name, telephone number and ID number but she said she could not take my complaint unless I confirmed my cell number. This I did not want to do. I then spoke to Sindile the Call Centre Manager. I confirmed my ID number again, my phone number, my account number and my invoice number. He also refused to go any further without my cell number. I told him that that was not relevant to my account or complaint. Why can telkom not register a customers complaint using the account details and ID Number. Why is interest charged on an account when I have not agreed to this