text your phone from a mates phone and tell him he owes you big money and you're going to get it >:')

have someone that is your friend spy out your locker and set the bitch up. then call the cops. that's my 2cents

Envelope with $100 monopoly dollars.

Ok so Joke or not, this is a massive inconvenience, and for the note to be put on your work locker, it has to be someone at work who's done this. Unless you work somewhere the public could possibly access your locker, AND know it was yours....which is stupendously unlikely. In which case leave a note for the taker on your locker, stating a time and place to meet. Show the ransom note to your boss ( I really hope you've left it in place on your locker ) and state that this sort of behaviour, if a joke, is absolutely unnaceptable. You need your phone.You want the person identified and dealt with severely. Frankly I'm a tolerant person but even I wouldn't put up with this sort of "humour". I would want that person fired. If it IS a genuine theft and ransom , then leave the note stating a time and place to meet, and have your boss, and perhaps one police officer if possible, with you but hidden somewhere out of sight of the meeting place but close enough to see the person when they arrive. Then when they turn up, and I'm guessing they'll be stupid enough to do so, hand over the money and have your boss/police officer leap out on them. They'll be caught and fired/possibly arrested. A little OTT to get a phone back, but I guess that depends on how badly you need that phone, and what is stored on it.

Better tell your boss because at least you know that the person who stole your phone is your co-worker.

I would ask someone trustworthy what he/she thinks. You may be able to narrow it down to a list of a few suspects. Then let your boss know. If nothing turned out, I would then call my phone company and give them the IMEI number so they can block the phone, and buy a new phone.

maybe call the cops? or text your phone from one of those apps and figure out who it is?

Check the security cameras!!!

If it's an iPhone, use find my iphone on icloud and see if theyre stupid enough to leave it on. And tell your boss, he don't want you to go to headoffice with this as theft from workplace is pretty serious and could lead to the boss being fired/ getting a probation/ getting a fine ect since he/shes in charge of the workplace. (least i uk, don't know the rules in us)

Have your manager/boss help you use the note to compare to people's handwriting. Because phones are so expensive nowadays, it may be felony theft to steal one. I'm sure the person rather give the phone back than get arrested and carted out of work (like one my ex-co-workers).