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Glossary · Numbering

Caller ID (CLI)

In one sentence

Caller ID, or CLI (calling line identification), is the caller's number passed through the network and shown to the person called. A business may only present a valid number that it is entitled to use.

Also known as : caller ID · caller IDs · CLI · calling line identification · calling line identity · presentation number · outbound caller ID

What is caller ID?

When your phone shows who is calling, that number has been sent through the network along with the call. It is known as caller ID or CLI (calling line identification). For a business, it is also a decision: should calls show the main switchboard number, the user’s own DDI, or a department number?

In the UK this is regulated. Ofcom’s General Conditions and CLI guidance require providers to pass valid, diallable numbers and to block calls with CLI that is invalid or doesn’t conform to the numbering plan. The number presented must be one the caller is allowed to use and one that can be called back. British networks also distinguish the network number (identifying the line, used for tracing) from the presentation number a business may choose to display.

How it works

  1. On an outbound call, the PBX or cloud phone system puts the number to present in the call signalling (in SIP, the From or P-Asserted-Identity header).
  2. The provider checks the number belongs to the customer. Where STIR/SHAKEN or a similar scheme applies, it also signs the call to vouch for it.
  3. The network passes the number to the called party’s provider.
  4. The handset displays it, unless the caller has withheld it (141 in the UK, #31# on many mobiles).

Example

A property management company in Glasgow has 15 staff who chase tenants and contractors by phone. Each person used to show their own DDI, so callbacks reached people who were out on site. The company changes its rule: outbound calls about rent show the main 0141 number, which feeds a shared call queue; calls about a specific repair still show the property manager’s DDI. Missed callbacks fall, and no number outside the company’s own range is ever presented.

What to check

  • Every presented number must be allocated to you; numbers ported away or ceased will fail.
  • Marketing calls have their own rules on identification and consent; in the UK, see the TPS.
  • On your own PBX, outbound CLI is often set at two levels: the user and the SIP trunk.

Caller ID at Universal Telecom

On our SIP trunk you choose the presented number from those allocated to you, per user or by rule. The virtual PBX offers the same setting from its web interface.

Frequently asked questions

Can I show my main number from every extension?

Yes, as long as the number is yours. A PBX or SIP trunk can present the main number, a DDI or a department number depending on the rule set for each user or call type.

How do I withhold my number in the UK?

Dial 141 before the number, or ask your provider to withhold it permanently. Withheld numbers are still passed to the network and to emergency services.

Why do my calls show as 'unknown' or get blocked?

Usually the number presented isn't recognised as valid or allocated to you, or it doesn't match what the provider holds for your account. Check the outbound number set on the PBX and the trunk with your provider.

Related terms

  • STIR/SHAKEN

    STIR/SHAKEN is a set of standards that lets the originating carrier digitally sign a call to vouch that the caller is entitled to the number shown, so terminating carriers can detect spoofed caller ID.

  • DID (direct inward dialling)

    A DID (direct inward dialling) number, called a DDI in the UK, is a public phone number that rings a specific extension, team or call flow directly, without passing through a switchboard.

  • Geographic number

    A geographic number is a landline number whose area code points to a place, such as 020 for London or 01 for Dublin. It signals a local presence and is charged as a standard national call.

  • SIP trunk

    A SIP trunk is a voice-over-IP connection between a business phone system and a carrier. It replaces ISDN lines and carries calls and numbers over your internet connection.

  • TPS (Telephone Preference Service)

    The Telephone Preference Service (TPS) is the UK's free opt-out register for unsolicited live sales and marketing calls. Under PECR, businesses must not make such calls to registered numbers unless the person has told that caller they are happy to be called.

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