Untruthful Sales Agent

I was called by an MTN agent about a month ago to tell me that since my contract is about 24 months, I will am rewarded with either additional 50 anytime minutes or 1GB data at absolutely no EXTRA CHARGE (she emphasized that). I agreed to being granted 50 anytime minutes. To my surprise I got billed R100 more this month. I've called MTN twice today and every agent is directing me to a different department, this is ridiculous, I'm annoyed by the fact that the Sales Agent **** to me and now I need to spend my airtime calling the different departments to have this matter resolved.