Vodacom specials are a ****

Took a wifi contract that was on special in April for R499 a month for 24 months. Literally a month later I was being billed R514. I keep calling their customer service and no one can tell me what the extra R15 is for. I'm being told they see that I'm being billed R499 not R514. I have proof that I am being debited R514 but no one wants to hear it. I feel ****med and I am being told I cannot cancel the contract.