My dad used to switch off the internet/wifi at home and he was the only one who knew the password. This definitely limited my brothers' and my usage of the Internet. We didn't want to use the mobile data too much because we knew it would come out of our allowance if we exceeded the cap. It will be hard to control their usage outside and whether or not they will call back but you can win half the battle by limiting their usage at home by trying what my dad did. It might be a wee bit extreme but it worked haha.
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