atcomsystems.ca/forum Forums General General Telephone & Systems Information butt set phone number

This is what I need help with, Im just starting out with phone wireing I know there is someway when you use a butt set on a line that theres a phone number to call on the butt set that it will tell you the phone number your on. I just dont know that phone number, If someone know what that number is and can tell me please. thanks, matthew