I have a topup R500 airtime but it since I have a nokia Lumia, it runs out before topup date. i'm not sure why, is it something to do with my phone set-up?
If you're running WP8.1 check the app list for Battery Saver and see what's running in the background. You can turn off the apps there. Typically they'll be using the data.
Also check your email accounts in settings and sync to see if it's constantly checking for email.
Also if it's a concern you can get a data switch app you can pin to your start screen so you can turn data off.