Unexplained fees

I had a Vodacom contract which ended in December 2017. I phone Vodacom to cancel the contract as I won't be renewing it. I asked that the number be moved to a pre-paid number of which they did. Now, 6months + later I checked my ITC and Vodacom still reflects on my ITC. I phoned Vodacom to obtain a paid up letter as I need to update my ITC. My request was declined by Vodacom's accounts department saying that I still owe Vodacom a sum of R151 for a handset. This is very strange to me as my contract with them ran the complete 24 months and they moved my sim to prepaid. When I asked why Vodacom didn't contact me about this before I requested a paid up letter I was told "because it's a small amount". Till today I am still waiting for Vodacom to send me proof of the amount that I owe and explains to me this "handset fee". This is day light *******