Airtime disappearing from sim card

I am currently a cell c customer for the past ten years , for the past one year I started using a Samsung phone .I always had WhatsApp on my phone.since the 24/01/2016 I loaded airtime onto my simcard but few minutes the airtime disappeared.on the 25/01/2016 I have loaded airtime again onto my simcard and after a few minutes I tried to make a call and realized the I dnt have airtime ,and that's when I noticed that the airtime is disappearing frm my simcard.i called the called centre on the number 140 and spoke to a lady who said me that she checked on the system and noticed that the airtime jus disappeared so she said to me that she logged it an it will take 24 hours to get feedback.i received a reference number ***.and no feedback came.on the 27/01/2016 I noticed the same problem happening so I decided to call the call centre and spoke to a lady by the name of gugu , I she was not helpful to me at all .she was rude and all she says to me is that I used the internet for WhatsApp .I even told hr that WhatsApp does not take so much of airtime and she cut the call on me .pathetic service