Inaccurate invoice

I'm on Mtn Anytime 350 with Autopage and my monthly charge is R 336.40 but Autopage has consistently been invoicing for over R700 every month without explanation. I do not exceed my allocated minutes and they even carry over into the next month so it's a mystery why the bill is so high. It is impossible to get through on their call center and complaints on the website get no response.<br>