Call centre agent *****

I had a contract with autopage which ended last month and their call centre agent phoned me and told me the have a 2 Gb data for R120pm for 24 months which I agreed to take. I received the invoice and it was correct this month the invoice changed to R150 when I call to ask they are telling me the contract is 150 pm so their call centre agents are lying I am tired of autopage ****** I want to cancel their stupid contract I didn't agree to this