I would call the cops in on this one. There are two ways this could go: When the thief sees the cops are involved and realize you're not going to joke with this bastard, he/she will return the phone out of fear. Or, if the cops don't see it as that big of a deal and they can't find it by tracking the signal, at least file a report that way your insurance will cover it, since most companies will require a report about the missing phone, anyways. If the latter happens, have the IMEI blocked so that it is unusable, and the thief will basically have nothing more than a hunk of plastic and metal.

Tell your boss. Unless it is the boss. Then you can tell the cops.

put the note in a zip bag, phone police and ask them to get the fingerprints from it. Half serious there, if it is stolen property.

Okay, in general you should always set up the security settings to make you enter a four digit number (NOT YOUR BIRTHDAY OR THAT OF YOUR FAMILY) so no one can make unauthorized calls after they steal your phone. Pay the bastard off and then (after you have your phone back) tell your boss, other people may be victimized next otherwise. I assume you want your phone back otherwise I would go through the police/boss route first. There might be surveillance cameras, etc. in the hallway by your locker that can be examined so that the thief is caught. Good luck!

if you do get it back and you know who it is i think you know what to do next buddy, op.

find the person hold their stuff or them for ransom.

Call somebody. Odds are the jackass will keep both the money and the phone.

That really sucks! How are you supposed to know who to give the $100 to anyways? Im not saying pay up, just do a little investigating.

Take the note to the police and maybe they'll find some fingerprints on it.

Lol if it is a smart phone just contact the carrier