MTN sending debt collectors even though they confirmed that i owe nothing

I have cancelled my contract with *********** MTN with much difficulty. It has taken me months to cancel the contract because MTN is extremely unprofessional and unreliable. After many hellopeter posts and calls, it was cancelled. For some reason now NUdebt collectors are phoning me saying that i owe them money. I have an email from MTN to prove that i owe nothing. Please sort out this mess.