Confused and unhappy

I am sooo annoyed firstly, secondly confused ,how is it that i can be in arrears with my account if i have to put money in only to use the money ive been putting in the account in the first place ?? According to me its like a pay as you go thing if i pay the account R100 a month and not use it for 4 months there should be R400 credit for me to use?am i correct? Now how if i go there.. i use the credit i paid for 4 months AND pay in the balance HOW AM I IN ARREARS ????please i need somebody to explain it to me . This is ridiculous ,no body picks up the customer care line when ive tried to call i tried at least 3 times,

Just the other day i went to use the credit infact it was on fathers day so anyway it was about R400 +- i had to pay in some money and paid it cash .. so again how can i be in ARREARS