Data and airtime depleted

It happens occasionally that data just expires way before the expiry date. I also receive annoying messages that tell me it will expire and I should recharge. However mtn assures me this is just marketing but won't cancel. They tell me I've used the data when I know I haven't. Yesterday r360 and 600 mb just disappeared. I've checked data counts on my phone that say 700mb but usage per app don't add up. Got cut off while call centre was logging my complaint. So convenient. And obviously they don't call back