DMC harassment

I had a contract with CellC that was handed over to 2 debt management companies which is DMC and Nimble group, so I got a payment arrangement with both companies which I was offered a discount from both sides and then I decided to pay the lowest one which was the Nimble group and then I got a paid up letter from Nimble group which I've sent to DMC as they requested from me so that they can remove my name from their system which they refused to do despite me showing them proof that the account is paid up, they keep calling me almost everyday wanting me to pay them for an account that I already settled with another debt management company. I think they just mad that I didn't settle with them which is really ridiculous.