Bad customer experience

I lost my contract phone which was not insured and I went to the telkom in Waterfront to see if I can get another device, the lady by the name of Sibongile Makalima assisted me and told me I can do a early upgrade and still pay for the remaining months of my lost phone monthly with the new installment until the old one expires and the new contract would carry on, and I was comfortable with that. And now I received my monthly bill which was showing a device obligation/carryover with the amount of R5866 which is due month end January 2020. When I called telkom customer service to help me understand the device obligation carryover, they told me when I do early upgrade I have to pay the full amount for the outstanding month's of my old phone, but the consultant that assisted me never explained like that. If feel that's unfair for me as a customer because I could have checked my other options. Hopefully telkom will look into this, it's such a disappointment!!!