Microsoft's template site for Office includes a model contract which you can easily modify (it does need some Australian localisation to remove the word 'Mom' for a start). In the sample "Brendon family contract", there almost seem to be more reasons to take the phone away than possible times where it could get used, but quite possibly it's written with the bitter voice of experience. If you've tried this approach -- successfully or unsuccessfully -- tell us what worked in the comments.
Incidentally, if you are planning to download, take note: downloading templates from Microsoft for Office basically doesn't work with anything other than Internet Explorer, which makes a slight mockery of the company's argument that it has become more browser-agnostic.
Cell Phone Contract With A Teenager [Office Online]