Telkom refusing to end contract after 24months.

I have a 24 month cell phone contract with Telkom mobile that o took out on the 12/12/2015. There service is horrible and there signal is even worse. After a year and 6 months of fighting with them due to the fact that I don't get signal at home or at work which I checked with them when I took the contract out and I was told I have full coverage both at home and work. In September I was called and asked if I am aware that I can upgrade. at which time I told the consultant that I would not be renewing my contract due to the sh*t service I have received. Now when I call them in the week to make sure that as of the 13/12/2015 that I go over to pre paid so that I can migrate my number to a different network and now I'm told that is not possible because I have not sent them a cancelation letter as yet and once I have done that it will take 21 business days to process. ***?????? It takes them 25min to approve you and sign you up but your 24 month contract and 21 days to cancel it. This can't be Legal.