Caller ID

[Koliedrus]

I get a couple of these hang-up-calls per month.

Name: Tennessee
Number: 999-0000

It's infrequent enough for me to not think about.

Well, now I'm thinking about it.

[Billyman]

http://www.lortondata.com/cgi/telephone.asp

[Billyman]

Well dang. I thought the information provided would give you something else to think about. See if I look for stuff for you anymore. :p

[Koliedrus]

Settle down. I just got here.

Nope. I'm still wondering. Thanks, tho.

[Escape Artist]

Caller-ID is a Telco offering that is a byproduct of (C)LASS services. In this case, only those numbers reported by participating exchanges are returned, exactly which are and which are not is currently (March 1994) at the Telco's discretion.

Currently there are two types of Caller-ID. The first (often
referred to as "basic" service) just returns the calling number or an error message and the date/time of the call.

The second ("enhanced" Caller-ID) also may return the directory information about the calling number. At a minimum, the name of the subscriber is returned (the subscriber is not the same as the caller, the phone company has no way to determine who is actually on the line).

Caller ID information is provided as a 1200 baud, 7 data bits, 1 stop bit data stream usually transmitted following the first and before the second ring signal on the line. Note that this is not a standard Bell 212 or CCITT v22 data format so a standard modem will probably not be able to receive it. Further, the serial information exists as such only from the recipient's switch to the callee's location. Between carriers the signal exists as data packets.

The signal is provided before the circuit is complete: picking up
the receiver before the data stream is finished will stop/corrupt
the transmission.

Currently there are two types of information returned: a "short
form" which contains the date/time (telco and not local) of the
call and the calling number or error message. The "long form"
will also contain the name and possibly the address (directory
information) of the calling phone.

The "short form" stream consists of a set of null values,
followed by a two byte prefix, followed by the DATE (Month/Day),
TIME (24 hour format), and number including area code in ASCII,
followed by a 2s compliment checksum. Most modems/caller id
devices will format the data but the raw stream looks like this :
0412303232383134333434303735353537373737xx
or (prefix)02281334407555777(checksum)

A formatted output would look like this:
Date - Feb 28
Time - 1:34 pm
Number - (407)555-7777

Can a Caller-ID signal be forged/altered ?

Since the signal is provided by the local Telco switch and the
calling party's line is not connected until after the phone is
answered, generally the signal cannot be altered from the distant
end. Manipulation would have to take place either at the switch
or on the called party's line.

However, the foregoing applies only to a properly designed CNID
unit. For instance the Motorola M145447 chip has a "power down" option that wakes the Chip up when the phone rings for just long enough to receive, process, and deliver the CNID signal after which it shuts down until the next call.

Should this option be disabled, the chip will be in a "listen
always" state and it is theoretically possible to "flood" a line
making a vulnerable box record successive erroneous numbers.

I have received a report of a device called "Presto Chango" that
can transmit an extra ADSI modem tone after the call has been
picked up that will cause a susceptible box to display the later
information. It was also reported to me that CNID boxes marketed by US-West as their brand and made by CIDCO have been used to demonstrate the "Presto Chango" box.


Often a company with multiple trunks from the Telco and their own switch will report a generic number for all of the trunks.

There is a defined protocol for PBXs to pass true CNID information on outgoing lines but it will be a long time before all existing COT (Customer Owned Telephone) equipment is upgraded to meet this standard unless they have a reason to do so.


It's a bit old, what I found tho. :dunno: