Billed for six months after terminated contract

In December 2014 I started the process to cancel my MTN 2gig contract. I was notified via email mid-January 2015 that this was done. In May 2015 I realised that my account was still be debited and not only that, the monthly fee had increased. I sent an email straight away. It's now been a month without a response to my email. I tried calling 809 and 808. I spent 15 minutes on each number and the call just disconnected. MTN seems to have very patchy service delivery.