Telkom a bunch of *****

So I signed up about 2 years ago for the 20gb LTE deal, then last year I was called and asked if I would like to upgrade my account to 30gb+30gb for R 559p/m so I agreed. Now I realise I have been charge R600 every month since the upgrade to "pay for the device that was included in the original contract", so I have been charged an additional R50 for about a year to pay for the device which was never stated or agreed to when they offered me the upgrade. I checked the website and the 30gb+30gb deal costs R559 p/m so why am I being charged extra for the device on the original contract. As I did not receive a new device when I upgraded so why am I being charged so much? When in contact with an agent the agent disconnected when I asked the same question.