Packet Switched Data Calls?????

More than a month ago I received a shocking invoice from MTN in which I was billed R969.69 for Packet Switched Data Calls. I've tried phoning the customer service centre, holding on for more than 20 minutes, and I've sent numerous emails to [Email Removed] trying to find out what Packet Switched Data Calls are and how it's possible for me to spend so much while I have a data bundle and usually access the internet via wifi. I also requested for a credit limit to be placed on my account and asked how to access itemised billing. An amount of R1627.94 was deducted from my account in June without any MTN representative contacting me regarding my complaint. MTN, your service is appalling and I will be lodging a complaint with ICASA if you don't sort this out.