Wrongfully charged cancellation fee

I took out a phone contract with MTN and requested my prepaid number be moved to the contract because it's a hassle to have a new number. The phone arrived with a new number and I had to cancel the contract I was told I had 10 days to return the device to not be liable for any cancellation fee. It been a month now since I was wrongfully charged a cancellation fee of over R15000 and MTN is not removing the fee from my account, I can't take out another contract with them or any other company because they refuse to remove the fee which they have told me was incorrect. I'm literally stuck I've emailed their complaints department and I'm being ignored I ask to be transferred to their credit and billing department the call gets dropped every single time.