Vodacom are daylight ******s

I took out.a dual deal contract for R659 pm. I was phoned and it was suggested I take out insurance on the 2 devices, which I did, making the total R890 pm. Now I get an invoice saying I owe R1344. When I queried why they tell me it is an open contract. Never Once was I told you get and open and closed contract and if I wanted an open contract. When I asked the lady if they can change it, she tells me No. Why not?