Billed twice for a service I have not even used

Not happy with rain was sold wifi from rain by an agent by the name of Sebonelo telephonically I paid a prorated amount as the package I took was R595 come this month I'm billed R1198 and I'm told I need to pay upfront no one is willing to listen the customer service agent sounded dumb and did not know her story as she kept saying that's how their billing structure is set up how do u bill ppl up front for a service they have not used yet, she has put me on hold for 38 minutes refusing to get her manager as I will not be charged twice