Constant overcharge

I have endless problems with Vodacom. every month they charge me for two handsets. I have been complaining since June month. October was the only month that I was not charged twice as I took it up with Rudi, He assured that it will not happen again. On Friday 1 December I called the call center and was advised that I will be called back. reference number 1- ********** ********** . to date no one has called and my call was closed without it being resolved. I am being charged for two handsets on a monthly basis and demand I be credited or I shall take the matter further. I am only in possession of one handset as I returned the other handset in June and also have the proof that it was returned.