BAD UNCEPTEBLE SERVICE

I have sign up for contract on the 11 may 2019 they told me that they wont debit on the 20th of may, they told me about prorate they told me that I will pay R770 on a monthly basis and I agree and I told them that I don't want a open contract they said they put a limit I wont pay extra I was happy with the service, now I was shocked to see the invoice which said I am overdue with R1725.50 which I don't understand I called in to Telkom I spoke to Andile she told me that the contract was an open contract I was shocked because I never sign for that and they never advise me that its an open contract I wanted to lay a complain with them she said she cant assist I have to go to branch I explain that I don't get time to go to the branch she told me that she cant assist. kindly assist I want to pay what we agreed on.my contact details is *** and I am still waiting for debit order to go off DO NOT DEBIT MY ACCOUNT EARLYER OR AFTER THE 20TH OF EVERY MONTH