Unable to cancel contract via call center

I have been trying to cancel my phone contract since last month but still not getting the help I want, Went to the store and they told me to call the call center(135), Calling the MTN Call center is a waste, they pick and hang up or the agent will just pick up and don't say anything and you will just hear the other agents talking at the background, One time they promised to call back but they didn't.MTN must find another way for us to be able to close our contracts, this call center calling is not working