Billing dispute

How am I expected to pay for a fee I was not advised of? I just started with MTN on the 17/04/2023 the lady i spoke to said i will pay pro rata and May's invoice at once and my next bill will be in June. Why am I receiving an invoice telling me to pay R1230. I've already paid Close to R800 and i will not be paying any other fee i was not advised of.