600Gb data for 36 moths at R399, it is not

In December 2024 I took out a contract with Vodacom, 600Gb Data for R399 for 36 month. only to find that in March 2025 they debited my account with R442 and some change. Now I would love to know how can you have a contract with a company for the amount of R399 for 36 months and then on the 3rd month into the contract they charge R442. How the hell does this work. Vodacom does not stick to their contract, they do just as they please. This is just outright theft. I am disgusted with them. How can they just do what they like