BILLING INCORRECT!

I cancelled my landline and ADSL on 2 April 2015 (one month's notice as required). I paid my bill in full at the end of April. I received an invoice today for subscription for May 2015 due on 26/5/15 for R499!! I cannot believe this. Phoned in to the call centre, 38 minutes later, the operator told me it is because I cancelled on the 2nd of the month and not the 1st that is why I must pay for May. I said it is nonsense and then she said but if I go to a Telkom Office they will work out the pro-rata and then I can pay the correct amount. I said but why can't she give me the correct amount over the phone and she said no, she can't. I must go to the Telkom office. Her solution was I must pay the full amount and then Telkom will give me a credit. Yes right, a credit to use on what?