Assist me please

I am not happy with the treatment i got from MTN , August 2020 i bought a phone on contract for 24 month that ended this year .so after 24 months i went to the mtn store to cancel the contract and they told me they can't help me i have to call 135 i have been calling that number and aslo *** but no luck and today the person who was helping me just hug up on me after waiting more than an hour on a call .mtn still sending statement month to month and now they say i owe them R900 is been 3 months trying to get help and even at mtn store they don't want to help me . mtn at mams mall . Please assist me