Unauthorized upgrade done

I've had a data contract with a modem with Vodacom on a 24 month basis. Long after the two years passed, I've upgraded my phone, not the modem. They messed up my update on the phone. When I tried to resolve this with them, they upgraded my modem on their own decision. As soon as I realized this I've cancelled the whole contract. I've never received a new modem. Now I see that Vodacom have billed me R170.35 for the upgrade on my modem. I've e-mailed customer care twice and still haven't heard anything from them, to phone the call centre are very frustrating, they just put you through from the one to the other. I want this amount to be credited since I never authorized this upgrade!!