Overbilling

I won dstv rewards previous months and i received my rewards as promised but then this month i received a bill of R1245 which i dont know why but i believe it was caused by their rewards and now the expect me to pay that amount. Ive been contacting them since the 1st of this month to solve this but they never did, they always promise to solve but they dont, today i paid R592 because its the amount im supposed to pay and my account was due yesterday so now i still have overdue amount of R625 which i wont. pay because i dont know it please help