Harassing emails requesting payments when I pay monthly

So a couple of times now I've been receiving emails from this company after I've made a payment towards my debt with mtn. They keep asking for payments in order to avoid additional costs,they say. I've been responding,letting them know I've paid but this must stop,why demand a payment two weeks after a payment has been done?They usually ignore my responses & ask about the next payment. I've told them that this is harassment. Why they don't focus all this energy on sending me statements for transparency, I have no idea.I have to keep asking for a statement.