Billing Sky High

From June 2015 to October 2015 my cell phone billing never went over R 840 per month, in fact it averaged at around R 692. In November I got a bill for R 1711.32 and December got a bill for R 2567.19. I have phoned Altech Autopage and have been told that the billing is correct. What I do not understand is if my account is capped at R300 how my billing goes as high as this and I have not received a satisfactory answer for that. My other issue is that as from the end of January Altech Autopage will cease to exist and my cell phone account will go directly to Cell C. Until I have these charges fully investigated I will not be paying the outstanding balance as I do not accept these charges. I hardly ever make calls on my cell phone and during the week I am on wifi, so I need a proper explanation of how Autopage come up with these extremely high bills all of a sudden. This is totally unacceptable!!