Impossible to cancel contract

I'm trying to cancel my data contract with MTN. I emailed customer care and asked for the contract to be cancelled. They did not cancel it and asked me to email retentions or to phone them on 135. I emailed them and nothing happened. I phoned 135 from my MTN number and the call got cut as soon as I finished the prompts to get to retentions. The same happened from my Vodacom number. How am I meant to cancel this contract then?