mtn airtime disappearing

In october 2018 I had experienced airtime disappearance on my mtn number. First i recharged in order to make a short call but after recharging all the airtime was gone. When called the mtn call centre i was told that i had used mtn extra time (I dont know what it is) and i borrowed R15 worth of data. I use WiFi on the device and never use data but this time i am told i borrowed. I asked the call centre agent to cancel that extra time but unfortunately it could not be cancelled as i was told that i owed mtn the balance of what i had borrowed. I had to buy more airtime so that mtn agent could cancel that extra time thing they were talking about. 2 weeks after that, i bought R7 airtime to make a quick call and only spoke for 28 seconds and over R6 was left. this airtime had disappeared on the 31 Oct when i wanted to make another quick call. i called 136 and spoke to an agent (a lady) who kept on putting me on hold while checking what was happening, she told me that i had subscribed to a service but she could not tell me what it was. i asked for the amount being deducted but she put me on hold again and this time for more that 10min and eventually the call was transferred. Then i was getting a message that the mtn call center was closed and i should check the following day. I called again and spoke to another lady who said she could not see any records as the system only showed history of that day (31 oct). The lady told me that her colleague who was helping me might have cleared the history when i called earlier. Ultimately no one could tell me why my airtime was disappearing. I had to search on the internet what this mtn extra time is and the T&Cs and realised that i did not qualify for it because i hardly recharge that device. If i do, it max R7 per 2 weeks or a month. How then did mtn allow whoever activated that? I did not know how that worked but mtn is continuing to swallow my airtime. I hope one day all mtn who experience this can switch to other networks.