Poor communication

A couple of months ago I was contacted by Vodacom & advised that I qualified for an upgrade, which I did & entered into a new contract. For the last two months I have been charged two handset installments & on contacting 082135 I was advised that I still need to pay for the old handset until that contract ends, why was this not disclosed when i was phoned & offered an upgrade. Had I know this I would not of done the upgrade.