Highly Upset Loyal Customer

Good day. I have been a very loyal vodacom customer since September 2013.. In the beginning of this year I paid my installments up until the end of my contract which is until 31/07/2019. I visited the 4u branch in Secunda mall, and the kind lady assisted me well. Whilst in the branch I contacted vodacom stating that I am paying my installments ahead but they should contact me as soon as it expires so that I can either decide upon upgrading or going over to pre-paid. To my surprise I suddenly get phone calls 9 o' clock at night saying im R680 in arrears and that I am going to be handed over to lawyers... upon visiting the brach today, a gentleman assisted me by phoning h/o only to be met by a very rude woman named DUDU. She didn't bother listening to me and when I asked to explain she sarcastically stated "well done" when I was busy telling her that I paid my contract up until its end. She also made me understand that it is my responsibility to have cancelled the contract whereas according to what my sms said my contract is finished end of July 19... Also how can I be in such a high arrear amount if it is today the 12th of Aug????? My installment was about R350..... I REGRET having taken out a contract with them as I got treated horribly and I was made to feel that I am nobody just because vodacom didnt do their work. I want this issue resolved immediately as I feel why should I pay this R680 when I paid nearly R2500 finishing the contracts installments???