Sending and charging for a product I did not order - now cannot return

I received a phone call from Buy Mondo a Vodacom outlet. They told me that they can change my contract and I will pay R50 less to receive and extra 3 gigs a month. I would receive a total of 5 gigs for R50 less than I am paying now. I stated in the conversation I am not agreeing to this deal, and I was told I must not worry they will send me paperwork to read more about the deal! (the conversation was recorded so all of this will be on tape) Next thing a box arrives for me which I sign for. When opening it its a pocket wifi - which I didn't order and I don't want. Now when phoning Buy Mondo and Vodacom to try return it and try find out why they sent it in the first place - I can't get through on the phone number and no reply to my email. Vodacom have no idea what I am talking about. I managed to talk to someone from Buy Mondo yesterday and they were clueIess and promised to get back to me which they havent. I dread to know what I a will be charged for this device. How do I get response and how do I return this package!!