KB101 - How Caller ID Works


What exactly is Caller ID?

Caller ID, also known as CNAM, is a convenience service that provides the called party with the number and name of the calling party, if available. While the term "caller ID" technically only refers to the delivery of the calling number, it has become interchangeable with CNAM, which includes delivery of the caller’s name.

Where does calling number come from?

The calling number is provided by the calling party’s switch. With a traditional analog line this will be the actual number registered to the line. In the case of a PRI or SIP connection the calling party’s phone system can control what calling number is sent on a call-by-call basis.

Where does the calling name come from?

Determining the calling name (in the United States) is the responsibility of the called party’s terminating carrier. For each call they will do a look-up, or dip, into a CNAM database to determine the name that is currently registered to the caller’s number. Not all carriers reference the same database — there are several CNAM database providers, and the terminating carrier will typically contract with one of these providers for CNAM service.

Additionally, a PBX utilizing SIP trunks can provide a name along with the number on outbound calls but most carriers, including Clearfly, will ignore this name and dip into a database to determine the correct name.

What types of name registration are available?

By default Clearfly registers all numbers with its CNAM provider. The name which is registered is a 15-character version of the customer’s name. This can be modified to something of the customer’s choosing provided that it is no longer than 15 characters and only includes characters from the ASCII printable character set. For greater compatibility Clearfly recommends that the registered name only include letters and numbers, as some legacy systems have issues with punctuation characters.

Customers can request that a specific number be removed or withheld from CNAM registration entirely. The result of this will vary by terminating carrier, but commonly will show up as "UNAVAILABLE" or a representation of the city and state that the number belongs to, such as "DENVER CO".

What about blocking name and number?

Some customers, such as lawyers and collection agents, wish to withhold both their name and number from the calling party. While this is something that can be configured, most customers end up going back to sending a number, as many people ignore or flat-out reject blocked or anonymous calls. Below are the options available for blocking based on the service type.

  • Analog Line - Caller ID Delivery is blocked via Feature Codes

    • *67 + Number (as you would normally dial) - Block an individual call.

    • *85 - Block all future calls

    • *65 - Remove block on all future calls

  • PRI/SIP - Per-call control must be handled by the customer’s phone system

    • Simply do not send a calling number. Clearfly will interpret this as the customer wishing to not send a number and Clearfly will send the call as anonymous.

    • Blocking can be sent on the PRI, but this will affect all calls from all users. This is not common, so contact Clearfly support if you’d like to discuss this option.