cancelled insurance but they keep on charging me

I cancelled my insurance on my cellphone contract, its a samsung galagy s5, about two months ago, the cancellation happend telephonically. Now recently I received a invoice for the cancelled insurance. When I phoned them to find out what is going on, they said that it doesn't show on their system. So I believe the person I spoke to, never captured the information. Now mtn is sending me messages threatening me with legal action if I don't pay, but clearly its a mistake on their side. I tried calling them again but was put on hold for atleast half an hour, when someone finally answerd they put me through to who knows where but then I was kept on hold for another half an hour. After that I put the phone down and never called again. The mistake is clearly on their side but they keep on harrasing me now. I will not pay for that insurance because it was cancelled more than a month ago. Mtn is the worst service provider and I wouldn't recomend them to anyone!