Angry customer

I sold my vehicle and I notified Cartrack about that and made the arrangement to pay the cancellation fee. On the 31st of October they sent me a link via their platform on my WhatsApp, where I should pay. I paid as per our agreement on the very same platform. They sent me a messages saying I still need to pay my account. I asked them to check with the Account Department. And they keep sending messages that I did not pay, whereas I did. I even sent them proof that I paid. I don't know what more to do, because it seems like noone wants to get hold of me.