WHERE IS MY AIRTIME??

On the 5th April I loaded R12 airtime and when i wanted to buy data i got an insufficient fund notification. I found it quite strange but I just didn`t have the energy nor the time to call your contact center and query this. On the 21st April I loaded the same amount (R12) and wanted to buy data when I received an insufficient notification. When i checked my balance I found that I have R5 left. I have switched off my mobile data, have not made a call but my airtime is short, can some explain to me why you guys keep taking certain amounts of my airtime. This is really frustrating because I load data for a specific amount because I know it will sustain me for X amount of time. Perhaps as a consultants from Vodacom you might deem this as a petty query considering the fact that I bought a R12 voucher however for me this is integrity issues because how can you guys keep deducting my airtime without any notification. This is appalling customer handling and I refuse to accept it. I work for a reputable company and I am a customer care team leader so I would like to think that every query , no matter how small is taken seriously otherwise I can very well take my business elsewhere.