I have a big problem with MTN every time I buy airtime it takes my whole airtime and um left with nothing. I have been using this number for more than 10 years now and the thought of changing ****s me

I loaded airtime on Monday the 9th of October 2017 I loaded R80 and when I was about to buy bundles it says insufficient airtime I called the damn call center and they told me about background applications that's are running I asked him how is that possible because um connected to WIFI. The agent didn't help me in anyway. I again loaded airtime today which is the 14th and I used my 1800 points to buy airtime and bought R20 airtime and it's gone all of it. Um not going to call the call center again because I know I won't be helped so I am taking this platform to get helped. I can't live like this. I will port my number to telkom if this issue is not resolved. And trust me I don't wanna do that.