Contract ended but still charged for handset

My contract ended on the 18th Dec 2015. I called cancellations to request for my number to revert to prepaid at the end of December 2015 and confirmed when my last subscribtions will be debited and I was told end of Dec 2015 and will pay for any extra usuage I made in January 2016. Now I receive a bill to say I owe for the handset. If I paid 24 months for the handset, why am I being billed now. Made many attempts to try and find out through Vodacom but no one knows why. If my account gets debited, I will reverse it until I have feedback. This is not acceptable that no one can help me.<br>