Yes it sounds stupid to tell, but, if you think about it what would you have really done if someone took your mobile and didn't want to give it back (remembering you're in an office environment where everyone's watching). You wouldn't start a big argument over it and start shouting at each other, that's just as bad as telling on each other, and you certainly wouldn't have a scrap over it. What would YOU do if he took your mobile. Other than calling the police....
When boys and girls grow up and go into the BIIIIIIIIIIIG wide world of work.... you have these things called social skills. You would use these skills to sort things out like an adult, not resort to telling tales.