Shocking bill and even more terrible service from MTN

I was roaming in Zim from 6 Dec 2020 to 6 Jan 2021. To my surprise, I got a total bill of over R14 000. I have asked MTN to investigate this matter urgently since I was not using the phone that much...and I was always loading airtime to make calls since I kept being told that I have insufficient balance to make calls ( I guess this is because I had reached my contract limit of R1000). So how is it possible that MTN bills me that much money when in fact their phone was telling me that I couldn't make calls unless I top up?? More worrying is the fact that I requested for itemised billing over the phone on the 17th of January and was told to send an email to ***h I did. Up to now there has been no response except to say they are still "investigating". Yet they keep debiting my account. MTN, I am a tiny SME. You are bleeding me dry. Resolve this ASAP if there's to be any hope for this relationship to continue....otherwise consider yourself having lost one more customer to Vodacom, Telkom or even Cell-C!!!!! In case you are still interested, my number is ***.