Am so disappointed

Telkom Sucks... I pay R260 on my contract phone, on March it happened I missed a monthly installment due to some unforseen reasons, on April it happened that I repaid that March installment that I've missed. Now my statement says I should pay R740.... When I call call center they say if u miss a payment you get charged R200 n something which I was never told about.... Meaning am charged the same amount as my monthly installment...