Worst Service Provider!

Upgraded a contract 2 years ago, which should be ending in Jan 2018, however I was charged for my new device and old device from the time I upgraded which was Oct 2015, as 24 months is up my device is now paid off and I should no longer be charged for a device, I have been querying this with VODACOM for over 2 months , they are absolutely unhelpful! whenever I do get feedback the agents dont even know what they talking about, when I explain to them the issue, they are speechless. they still havent been able to give me proper feedback, from the time i started the contract, i have been having issues, they customer service is absolutely pathetic!