Mtn failing to cancel my contract

Mtn does not want to cancel my contract that I didn't even use. They delivered a home router instead of pocket router and I took it back to the nearest branch immediately (after three days of delivery) as I was instructed to do so. I then phoned to give them IRMS and request of contract cancelation. I kept on calling mtn to make a follow up since 14 June I phone almost every week but till today my contract is not cancelled and when I went to mtn branch I was told I got billed on the 27th of June and huge amount that I am not aware of.