Overcharging on Internet Contract for Laptop

I live in the Strand, Western Cape, South Africa, but I was in Tzaneen, in Limpopo Province, when my previous 2 year contract for 2 gig internet for my laptop, at R119 per month, paid by debit order, with a cordless modem, expired on 5/6/2015. My account number is A***.<br> <br> I went to MTN Tzaneen on that date and renewed the same contract, without taking a new modem or a new sim card. I was told that I would only pay R119 per month as before, for the next 2 years. I did not receive a copy of the contract.<br> <br> Since then MTN has been overcharging me every month by subtracting the wrong amount by debit order from my bank account. I have been to the Strand MTN outlet twice, aand they said they cannot help me. <br> act<br> On 1/6/15 they subtracted R126, on 1/7/15 R126, on 1/8/15 R252 ( 2x R126 ), on 1/9/15 R147, and on 1/10/ 15 R147.<br> <br> I have also sent a few e mails to customer care at MTN. They first said that a upgrade fee of R92.10 was charged, but if you ad it all up, then they still overcharged me.<br> <br> Every e mail has been answered by a different person.In the last e mail I was told that their fees have sgone up since and that is why I am overcharged. But can they just change a contract ?