Operation
In the United States, caller ID information is sent to the called party by the telephone switch as an analogue data stream (similar to data passed between two modems), using Bell 202 modulation between the first and second rings, while the telephone unit is still on hook. If the telephone call is answered too quickly after the first ring, caller ID information will not be transmitted to the recipient. There are two types of caller ID, number only and name+number. Number-only caller ID is called Single Data Message Format (SDMF), which provides the caller's telephone number, the date and time of the call. Name+number caller ID is called Multiple Data Message Format (MDMF), which in addition to the information provided by SDMF format, can also provide the directory listed name for the particular number. Caller ID readers which are compatible with MDMF can also read the simpler SDMF format, but an SDMF caller ID reader will not recognize an MDMF data stream, and will act as if there is no caller ID information present, e.g. as if the line is not equipped for caller ID.
Instead of sending the caller ID in between the first and second ring, some systems use a "line reversal" to announce the caller ID, or caller ID signals are simply sent without any announcement. Instead of Bell 202, the European alternative V.23 is sometimes used, (without the 75-baud reverse channel) or the data is sent using DTMF signalling.
In general, CID as transmitted from the origin of the call is only the calling party's 10-digit phone number. The calling party name is added by the consumer's terminating central office if the consumer has subscribed to that service. Calling name delivery is not automatic. An SS7 (or Signalling System 7) TCAP query may be launched by the called party's central office to retrieve the information for Calling Name delivery to the caller ID equipment at the consumer's location if the caller's name has not already been associated with the calling party's line at the originating central office. Canadian systems using CCS7 automatically (but not in all cases) send the calling name with the call set-up and routing information at the time of the call.
To look up the name associated with a phone number, the carrier in some instances has to access that information from a third-party database and some database providers charge a small fee for each access to such databases. To avoid such charges, some carriers will report the name as "unavailable", or will report the name as "(city), (state)" based on the phone number, particularly for wireless callers. For 800 numbers, they may report a string such as TOLLFREE NUMBER if the name is not available in a database.
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