Unfair Unprofessional Collection Action

I have had a cell c contract from 2002 until 2015. 13 Years in other words. I decided to upgrade over the telephone and had my handset delivered to me. The process was handled so unprofessionally, I had to phone back everytime. I eventually decided to not go ahead and gave Cell C my one month's notice of cancellation (Via Telephone as well as email). It was acknowledged by them via email as well as on their system. At the end of the contract term (October 31st) I paid my final dues as quoted by Cell C and they then gave authorisation to Vodacom to continue to port my number. NOW after 6 months I start receiving SMS's daily from their collection agency. They also phone me to request for payment of R112 and that they will give me discount if I accept. I told them my story that it is impossible because Cell C gave authorisation to port to Vodacom and I paid everything before then. I also requested proof of this but they told me that they only do the collection and have no information on the account. Every day I am threatened with SMS's requesting money and that I will be handed over if I dont pay immediately. They only give you a contact number and when you phone the number it only rings. I then started replying to the SMS's but off course there's no reaction ! I am very dissapointed in the way they threat me daily and they also deregistered me from their website account login.