In most countries, caller ID displays either show just the phone number or a name and a phone number. A phone service that provides caller identification with just a phone number is called single data message format (SDMF). Those that display both a name and number with the phone call provide what is called multiple data message format (MDMF). While the technical means by which a phone company sends information to caller ID displays may vary by country, it is usually provided in one of the two formats.
Both single data message and multiple message format provide phone subscribers with the date and time of each call. Some phone companies may provide their customers with caller ID displays and the equipment is considered to be owned by the phone company. This type of equipment is usually stand-alone and plugs into an existing phone set with a standard RJ11 phone cord. Many digital phone sets sold in industrialized countries come with built-in displays, eliminating the need for equipment rental from the phone company.
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Caller identification is displayed in digital format and transmitted as data over existing phone lines. In countries such as the United States, that information is sent after the line recognizes the call's first ring tone. With SDMF, the recipient sees the phone number the call is originating from. The recipient may choose to reject or pick up the call based on whether or not he recognizes the phone number.
When the recipient is away from his phone during an extended period of time, caller ID displays will still capture and store phone numbers. A recipient is able to scroll through the stored information and delete it from the display's memory as necessary. The date and time feature of displays allows the viewer to determine if the call was recent or left several days prior.
Multiple data message format is one of the types of caller ID displays that includes the name that is registered to the call sender's phone number. That name may be business or personal. With MDMF, some names do not display in their entirety. Likewise if the phone number was previously reassigned to a new subscriber, the caller identification may temporarily display the old subscriber's name.
Phone subscribers often use the name portion of MDMF to determine if they want to take a call; this is a form of what is known as "call screening." If it is a name that is not familiar or is a call from someone a subscriber does not want to speak to, he may let a voice mail service or answering machine answer the call. Caller identification was introduced as a popular way to avoid or screen out unwanted calls, specifically from telemarketing organizations.