Vodacom

I had a payment arrangement with Vodacom as I fell behind on my account. In October 2018 I received a statement with an amount due of zero, In the last few months I paid double to settle the account earlier. After paying the last installment I mailed the statement and proof of payment to them and requested a settlement letter, I never received the letter. I never heard anything from them until this morning when I received a mail "Broken Arrangement" I rep**** with a mail sending all the proof of October 2018 after that I received a FINAL LETTER OF DEMAND then I decided to phone as well. The lady that assisted me did not understand what I was trying to say and just insisted that I had to pay the money as that is what shows on the system. I was still trying to speak to her when she just said "Hold to rate the service" and she ended the call. Why did they not respond to my mail in October 2018? Why did I not see this on my statement? Now 10 months later I must find that I will be handed to a debt collector. Unfortunately I do work on a budget.