Debt collector for R20

I have had my Identity account for about 10 years. Last Thursday I had about 10 calls on my phone from 031 numbers. Eventually when I answered on Friday I was told she is calling from Identity. Needs to confirm my ID number. I told her I wont do that if theres a problem I will go to the store. They kept calling from the 031 number until Friday evening. When I answered again. I told the guy I dont know why they are calling as they have never called me in the last 10 years and I dont owe them any money. I checked my online statement it said R120 due by end October so I paid it as I could not understand how I owed them my balance was about R200. They guy very cheekily tells me thats not true. So I went to the store on Saturday by this time 5 missed calls. The manager tells me I owe the R20. I am completely confused about the manner in which they handle customers. Could the person not have told me when I paid last month its not enough? Couldn't identity send me an sms and tell me to go to the store instead they apparently hand me over to debt collectors to harass me for R20? This is just shocking