Well, it's a smartphone. So, airtime gets used when you have the Internet going at any time. It's never a good idea to get a smartphone if you need to stick to the pay-as-you-go method. If you can't afford a smartphone just wait until you can. I used an iPod Touch for 3 years until I could afford the phone contract. I used a TracFone for that entire time.
Airtime will be depleted if you actually use it. Check whether there are some functions that use airtime that might be running in the background. If you have used the internet, it might be the cause, but if you haven't, contact your carrier for assistance.