Here is my list of Phone Numbers found on walls, signs, posters, etc. To call a number: Open your phone, choose Contacts, press the X button, enter the number, press X to call.
None of these do anything except 911, which adds "Emergency Services" to your Contact list. When you call them you can choose to have a police car, firetruck, or ambulance sent to your location.
The rest of the phone numbers I've found are useless though a few have a short message. When I started searching for numbers and writing them down I was expecting to find some that were humorous, or maybe I would find a hidden secret... but so far nothing is even interesting. At least hunting for them is fun.