By airtime do you mean a call allowance (which is what I'd understand by the term) or a data quota? Of course if you are on pre-paid the two could be mixed together.
Something making calls when you are not using it would be very weird, so I'll assume it is data use. You can of course turn data off when not using it. Otherwise, I'd suggest checking the settings of apps which could use data, turning off any syncing for apps you don't use, increasing sync intervals if these seem too short (do you need the weather data updated every hour?). You could also install a data monitoring app: 3G Watchdog can show you which apps are using data, which will help in identifying the problem.