addition charges not explained

good day , my id *** i have made arrangemnts with them for R700 a month for my hyphen account , this month was my last payment i made paymnt R687.17 as this amount is what reflects as balances on their app, i made paymnt using incorrect ref now, i tried to contact them several times , first line is poor then i chat on whatsapp they disconnect my convo without even explaining why they now charging me R780 . i asked for reason for additional charges , im done foning them becz i get charged for contacting them