Airtime Vanishing

Am on contract so last week Tues I loaded airtime for R48.00 after I checked that I still have alot of talk time in minutes, data bundles and SMS bundles. To my surprise when I checked over the weekend my airtime was finished but the balance for talk minutes was still sufficient.<br> <br> loaded another voucher of R20 airtime the following week everything was ok. To my surprised yesterday my talk minutes were exhausted without any notification and whats funny is that when I called to enquire about this the call centre told me that I made a call around 08:05 and that is when my talk minutes were exhausted but thats not true because I was busy sending SMS' not calling. I requested to speaking to someone senior but I was told that the shift supervisors were on lunch at 19:15.