Mtn signed me up on a certain package but every month they debit me an amount that I was never told

Mtn I have been your customer for years but I am tired now. I want out. Your service champion signed me up for a data deal at a certain price but now another person from your call center is telling me that package doesn't exist. Now I am being told I can't cancel the contract and I must continue with the contract for 36 months on a higher premium that I was not told. How can you do this, is this even right and ethical??? Please cancel my contract