Legal action for non payment of an account that was cancelled.

I have cell account no. NM 06641-1. I ended my phone contract about 6 months ago by visiting their Rosebank store. (I paid by debit order...for about 20 years). I have since been using pay as you go on my phone ********** 475. I have not recieved a bill in all that time nor any attempts at deductions by debit order. Yet..I am constantly harassed by sms for 'not paying my bill' . I get the sms from Vodacom, I phone and explain. The call centre acknowledge that I am now on pay as you go. That should be the end of it. But its not. I'm messaged again next month. This month I am advised that I've been handed over to legal services. How do I resolve this. Ive been pro-active in getting it resolved yet one side of Vodacom would appear not to know what the other is doing. Will I soon be blacklisted ? I need help.