MONEY GETTING LOST ON MY PHONE

I am very upset I put R1000.00 airtime on my Phone and less than a week the R1000.00 is Gone I don't Phone that much and I don't use this phone for any other reason then phoning and WhatsApp , during the day I have WIFI and at night I have WIFI its only when I'm out that I use the data.

I don't even use the phone for games or fb.

VODACOM Please tell me where is my money.