You get into service mode by pressing the red button on the little metal bracket that has the volume control and sound amplifier. Open the coin door and look inside, it will be facing you.
You force a call, register, unregister, etc from the connection menu in service mode. I can't remember off the top of my head what the name of the menu is, but you use the left and right buttons to scroll up and down. Each menu will have sub menus. I think it is like the second or third from the bottom. Pretty obvious once you get in there. As soon as you enter service mode the first screen you are presented with will let you know if you have a cabinet id or not.