A number one can dial and be told what the phone number of the line you are calling on is.
Extremely useful in phone installations, but also greatly abused by fone phreaks. As a result, these numbers are generally kept secret, although your local phreak knows it anyway.
Listen carefully when your local telco man does a phone install and you are likely to hear him dialing ANI- remember those DTMF tones, and you can use it too. The numbers are generally local to each area.