Where did my airtime disappear to???

Please tell me where my airtime is disappearing to!

Last night, I loaded airtime of R12, as well as a data bundle of 100MB, to my number. I have not made any phone calls or sent any SMS messages since last night.

(I have only read and sent a few WhatsApp messages, supposedly using the data bundle.)

This morning, there is no airtime left on my phone! Where did that R12 airtime disappear to???

Please respond via email.