Vodacom harrassment and poor service

Vodacom has become increasingly annoying with their harrassment. I cancelled my contract in December 2018 and was told that I just needed to pay the cancellation amount and all would be in order. I paid that and thought I was now done with them. In March and April 2019 I saw debit orders go off my account, I reversed this as I no longer had a contract with Vodacom. Vodacom then proceeded to harras me with calls saying i owe them and when i explained to them my contract is cancelled, they then tell me well i must sort it out with retentions, amazing service first of all vodacom, i am very happy to do your job for you, NOT!!. Vodacom then called after i logged a Hello Peter complaint and then credited the account accordingly only to tell me well i still owe as a invoice was generated after my cancellation, HOW? THE CONTRACT IS CANCELLED, SHOULD I SPELL IT FOR YOU, CANCELLED. I then keep getting calls and each time I answer and it gets diverted and just rings, never to be answered, how poor can your service be and get, you are taking this to new extremes, please sort out your poor admin and do not harass people this way