Bad telkom

I have a mobile contract with telkom now my bill was R1499 that needed to be paid by Desember so i ended up paying R1700 leaving me with 101 in credit so this means that i did not needto pay again in December cause i paid my bill for Desember now i got January bill saying I have to pay R 1,547.50 by the 15th of Desember for January's bill i know the amount is correct cause I did buy 10gb extra but i did not budget for another bill in December since I was in credit till January for my next bill this is frustrating i always pay in advance till now 2 bills in 1 month they are killing us