Telkom

I have a 24 month contract with telkom iv been paying my account every month via peep as I do not have a banking account.I have called telkom on the customer line and queried why is my account balance going up every month as I'm paying my R449.oo as per my contract every month the gentleman said that I am in credit it's not a problem but this is now going up I just received my invoice again and now it's R308.oo to be paid by 31 Jan can anyone assist me pls cause this is not right as I am stressing to make that payment every month now I must be owing R308.oo and it keeps going up every month thanks in advance