FNB Airtime Disapears

I bought some airtime for my second emergency phone about 3 weeks ago (R10). I don't use this phone at all as I only keep it for emergency. Today I get a message from FNB stating that my airtime balance is low. I looked on my FNB app to see I now only have 1c left? Where the hell is my R10 Airtime ????? ****************!!!!