Billing Dispute

I have a Vodacom contract for an i-phone 6 that i took mid 2015. for the first few month i got charged some irregular amount because i did not have a call limit. i have since put in one so that i can control my spending limit. Since then i have been averaging around R 600 or so, but to my surprise a was billed over R 900 last month. I called their call center and they explained that when u have a call limit and it reaches zero while u busy on your phone it continues until a session is done. That is complete BS because my understanding of soft lock is that it should lock and nothing should proceed beyond that point.Over R 350 spent on \a session\" ??...i do not buy that for a minute...that equates to over 2 GB used. I have another contract with telkom mobile and unlike this guys you get a warning when you data reaches 99 MB and when it reaches 0 MB and it stops irrespective of whether you a busy on the phone or not. This to me is serious exploitation by Vodacom to milk every cent from their customer...I'm very disappointed in how you conduct your business...Not happy at all ...I want to know what the other R 350 or so was charged on and what exactly is the purpose of setting a limit?"