How to STOP you cancelling your phone contract with MTN, just never answer the call

Hi, I have been trying to give notice to cancel my MTN contract. I have phoned numerous times and after getting vetted I get put through to a very irritating monotonous tone. The shortest time I listened to this tone was 20 minute the longest was 30 minutes. Surely no company can be this busy and if they are then they should have more people answering the calls. Roger Wright