MTN & MONDO are ripping me off!

Took a data Sim by MTN from MONDO the sales person told me it's charge at R99 p/m and I was billed double the amount! I asked that they cancel the deal because the price that I was told had changed. MONDO said they ivestingating and would send me voice recordings, nevor got back to me even after numerous follow ups. I escalated to MTN as the continued billing me and they too said they conducting an investigation and never got back to me.. Im not using the Sim card, I requested it be canceled yet I'm still being billed. This is a total *******.