Cell c worst contractor

Cell c is the worst service provider ever, I have been paying them well with no problems. Now last month the wanted to charge me double. I then sent them all proof of payments that I did not skip a payment until today no one has assisted me regarding me matter. All they say is that they charge me penalties for late payment even when I paid on time they still charge me extra. Please help me solve this matter as I keep on reversing the other amount that I don’t owe them. I don’t want this to ruin my credit score.