Harassment from Vodacom

I am now getting very annoyed from these threats. Last year I phoned to change my contract to a prepaid as it was over the end of contract period. When I spoke to your call centre I asked her if I owed anything and she said no I do not, and if I did I was not able to get changed to a prepaid! She changed me to a prepaid.... 4 months after I get sms and phone calls saying I now owe Vodacom??? After mentioning the above numerous times the threats stopped now I get a sms today stating I owe above the original amount... How do I get put on a prepaid if my account was up to date then 4 months after the change I get told I owe Vodacom?