Vodacom is significantly overcharging me

Since I signed a new contract with Vodacom in December 2017, every month's bill has presented with new, unapproved charges. My package come with sufficient call minutes for my usage and I do sometimes buy a data bundle but my bills are ludicrous - they should be between R600 and R800 but last month was over R1500 and this month (so far) is over R1000. I'm so sick of these big companies cheating customers out of their hard-earned money through BS or undisclosed charges. Fix this, or I take my business elsewhere.