Price lock bungle up

on 25 October 2015, dstv debited my acount with R243.00. my subscribtion every month is R1050.00. After numerous phone to their call centre, every one told me that my acount is fine and that I have a credit of R900.00 that is why my debit was only R243.00. last week i found out that i am in arrears for R1500.00 wich must be payed.I phoned in again to query this and I spoke to the agents, telling them what really happened and their mistake should make me now. I eventually made an agreement so that we need to pay that money off in three months(knowing well that it is their mistake to begin with). I asked their fincial depatment to phone me so they explain to me i must now pay for their mistake, no one has yet came back to me(there are no numbers for me to call them i must wait for them to call me and they have a backlog). All i want is an explanation and a way forward