Upgrade extra charge

I use vodacom as service provider. I recently upgraded on date recommended by vodacom and on my bill is an additional charge for previous phone on top of upgard. I was never informed of this charge and have been with Vodafone for over 10 years which equals about 5 upgrades. I have never had this issue. I called and logged a complaint. I was informed that someone will call in 7 days in this time the debit order has gone off. I made a serious complaint on the phone and feel my hands are tied. It is a big amount and I am not willing to accept this? Anyone know how I can accelerate this? #vodacom #veryunhapppycustomer