Voip Blog
Dial Telephones, ANI, and Caller ID
Wednesday, May 28, 2008
It's funny that we still use the phrase dialing the phone when most people under the age of 30 have never used a dial telephone. I was using rotary phones well into high school. If you've never used one of these phones, this instructional movie short from 1927 should help:
Back then, you typically only dialed telephone numbers in your local area. If you needed long distance, you called the operator and they connected you. Most people didn't know the area codes of distant cities, nor did they need to.
By the time I was born, direct distance dialing, as it was referred to in old telephone instruction manuals, was fairly common. Even with direct distance dialing, there were some locations that required operator intervention.
Here's a crazy fact about dialing long distance calls: you had to supply your own telephone number to the operator! I'm sure this was taken advantage of by nefarious individuals, not to mention a source for billing errors! There wasn't automatic number identification (ANI) back then. One direct distance dialing became more prevalent in the 1960s, ANI simply came along for the ride.
ANI is something that is used within the telephone company trunks. Caller ID is a consumer-level manifestation of ANI, i.e. the transmission of caller information, but it's a completely different system from ANI. ANI is typically not seen by a consumer--and can't be blocked. Caller ID is seen by the consumer--assuming this service is enabled on your handset.
Caller ID can be communicated in a number of different ways, based on the local telco standards. In the U.S. and Canada, it is communicated as a short burst of modem data between the first and second ring. In the case of Call Waiting Caller ID (i.e. caller ID while you are on a call), your phone handset has to go quiet for up to 100ms in order to capture and decode the Caller ID!
