If I leave 3G on, on my phone, will it use up data?

[puffin57]

It will use up some data, since the phone and apps may check for updates from time to time, or things like Facebook or Twitter etc will check for updates too. It probably won't use up all that much, but if you leave mobile internet on all the time, then it all adds up.

Also, if the phone were to check for OS updates, then it can easily install them on the mobile if it finds one and this can eat up lots of data (I got bit by this when setting up my son's phone, I wondered why he had little credit left, and discovered that it had updated the OS via mobile - I had forgot to set the phone not to automatically check and download updates!).

What you are best to do is simply turn off the mobile internet when you are not using it. The phone will work fine, just switch on internet when you require it. Set your apps to manual update only - this can be done in the play store (android) and I assume this can be done on the Apple store too. (To do this, go to the store, select the app you have on the phone, and uncheck automatic update box. Do this or all apps, or when installing, ensure it is not set to automatic. This is for Android, but there probably will be equivalent for iPhones).

With the OS updates, go into settings - About phone - updates. Uncheck automatic update. OS updates are best done via wifi, since they can be pretty data hungry.

Use wifi wherever possible, too. This will save your mobile data allowance big time, especially if you like to watch videos and any other data hungry things like that.