CellC give me my money bac****!!!

CellC is ****. I have two running contracts with CellC and requested a settlement amount for my one contract. I then paid this amount but by their mistake it was an overpayment and they then credited my other contract with this overpayment. This was there mistake and I have been struggling to get refunded as I didn't request for the overpayment. CellC can you please sort out this mess.