MTN blocking contract cancellations

MTN tries to avoid at all costs to have my contracts cancelled. I am living outside South Africa, and MTN does not allow my calls to 135 to go through. I have emailed the retentions department on several occasions and still do not have any reply. I really just need to get the 2 contracts cancelled which I have patiently waited to run out and paid diligently even though I have not been needing it for the past 6 months.