I hate to be the party pooper, but...
Just because the phone number is "ringing" when you dial it, doesn't mean that the phone is on and ringing somewhere, although that would normally be the case.
My guess is that the phone is indeed off and in the possession of someone else. They may be either trying to get rid of it for money or trying to find a way to hack it for their own use in a way that would not get them caught. Who stole it? Anyone's guess. You said you're new at the office, so I'm sure you don't know the people well enough yet to realize who would possibly perpetrate such a crime. How much money a person makes doesn't preclude criminal behavior, just look at pro athletes!
Seriously, though, I'm very sorry for your loss. I think you need to chalk it up to bad luck and buy another one (guarding it with your life, of course!)
And this is definitely raising my awareness level for my own iPhone. Thanks for the heads up.