If they tried calling you and your phone is on, they'd be able to tell by the fact that your phone is ringing.

Just play by the rules. There are other ways to amuse yourself when you get bored I also have the feeling that if you were caught, it could lead to unpleasant results, especially if you're working with sensitive material. I'm betting you probably don't want to lose your security clearance. (I don't know what the criteria would be to lose that, but I'm assuming that that wouldn't help.)