Airtime being deducted when data bundle is loaded

I have a data bundle loaded and active, yet my airtime is being depleted instead of my data. I've called your call centre and been into your stores multiple times without a clear response of ANY SORT, I get told it's a technical error, they don't know why or there's multiple sessions open. I am in no way a novice user, I am a Apple certified engineer and developer, I am positive there is no multiple app or internet sessions, I only use 1 app at a time with background activity switched off, usually \Whatsapp\" or \"9gag\". Yet this week alone I've spent 300 rand alone, THIS IS UNACCEPTABLE. I've been into your flagship store, where your technical team had agreed with me in that there is no multiple sessions or anything wrong on my side. I need this to be resolved!!! If u insist on multiple sessions being the problem, prove it. If there's another issue, I shouldn't be paying for it. I need this resolved immediately!"