MTN's inability to resolve a simple billing query

I have switched to MTN from Vodacom mid February, due to connectivity issues in my area, and though I am otherwise happy with Vodacom had no choice to make the switch.

With my new contract with MTN (after 6 weeks, and 3 inquiries) no one has been able to give me a clear answer where the amount for my first payment comes from, and how it was calculated!

Surely this cannot be such a difficult question to answer and the issue is integral to their business - I would greatly appreciate if someone in the know can contact me, and explain to me how their mysterious billing system works?? The My MTN app, vs calling in for automated prompts, vs speaking to a consultant, results in 3 different answers......