CellC withholding funds not owed to them

Due to a payment error, CellC was accidentally paid a sum of money, instead of another account. I reported this error on 4th April and have kindly requested this amount to please be refunded as it is not owed to CellC and that this amount urgently needs to be refunded, in order for me to pay the correct account. I have provided proof of the payment error and have dealt with about 10 different agents, all who advise "we have received your query and you will be contacted in 24 - 48hours". My frustration is quickly growing as I am yet to receive a single follow up call from any of the 10 CellC agents, and my refund has not been actioned. CellC is *******ly withholding funds that are not owed to them and refusing to return calls, Ive sent numerous emails, logged complaints etc, and yet they still refuse to contact me and will not assist with refunding the money which is not owed to them! They have now blocked my number so I cannot get through on the Customer Care line helpline. Absolutely shocking.. this issue has been going on for 6 weeks - unresolved.