AIRTIME DISAPPEARING

My fiance and I upgraded our contracts in March 2015. Our Cell phone network is MTN. In July 2015 R300 disappeared from my airtime in just 1 day. I wasn't even using data. My contract is on my fiance's name so he phoned MTN and he was told that the problem wasn't on their side but they will investigate and even sent me a reference number. Our monthly airtime gets topped up on the 21st of each month. On the 22nd of August I noticed yet another R20 got deducted without me using my mobile phone. I rebooted my phone and noticed that took R40, \It's quite expensive to reboot my phone don't you think?"On Saturday 22 August I went to MTN in North Park mall"