Handed over to debt collectors without prior communication and still being contacted after the account has been paid, a year and a half later

I was handed over for an unpaid amount of R450 without prior communication about the outstanding amount that the medical aid did not cover. I was contacted by the first debt collectors 6 months after the hospital stay that the account was related to. After being contacted by these debt collectors that Life gave my details to, I paid the account and sent proof of payment. I was then told by these debt collectors that the account had been closed and did not hear anything more. I then started receiving calls from another debt collecting company a year and a half after the account had been paid and have been sent from pillar to post to try and get them to stop calling me. Today I was contacted by a third debt collector regarding the same account, two years after the fact. When I finally lost my temper, I was told to calm down and they just didn't pick up the payment. If this is the treatment I have received for an outstanding amount of R450, I cannot even imagine how people who owe more are treated.