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Glossary · Numbering

DID (direct inward dialling)

In one sentence

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.

Also known as : DID · DIDs · DDI · DDIs · DID number · DID numbers · DDI number · DDI numbers · direct inward dialling · direct dial inward

How a DID reaches an extension directlyA four-step flow. 1: a customer dials 020 7XXX 0112. 2: the carrier recognises the number as part of the company's DID range. 3: the phone system reads the dialled number and looks up its rule. 4: extension 112 in accounts rings, without going through reception.1 · The customer dials020 7XXX 01122 · Carrier networkNumber belongs to thecompany's DID range3 · Company phone systemReads the dialled numberand applies its rule4 · Extension 112 (accounts)Rings directly, withoutgoing through reception
With DIDs, each person or team has its own number, yet every call lands on the same phone system.

What is a DID?

A DID is a phone number that goes straight to a person, a team or a specific call flow instead of a central switchboard. In the UK it is more often called a DDI (direct dial inward); North American carriers and most VoIP platforms say DID. Either way, the idea is the same: a customer dials 020 7XXX 0112 and the accounts team’s phone rings, with nobody on reception having to pick up and transfer.

A DID is not an extra line. Every DID lands on the same connection to the carrier, a SIP trunk or a cloud PBX, and the phone system uses the dialled number to decide where the call goes. That is why a firm can have 100 DDIs on a trunk with only 10 channels.

How it works

  1. The carrier assigns the business a block of geographic numbers, often in a range of 10 or 100 consecutive numbers.
  2. When someone dials one of them, the carrier delivers the call to the business with the dialled number attached (in SIP, in the request header).
  3. The PBX looks the number up in its routing table: ring one extension, ring a group, or start an IVR.
  4. On outbound calls, a user can present their own DID as caller ID so customers can call them back directly.

DIDs can also have time-based rules: a support DDI rings the team during office hours and goes to voicemail at night.

Example

A recruitment agency in Dublin has 12 consultants who spend half their day on the phone with candidates. Calls used to come in on one 01 number and get passed around, and candidates complained that they never reached the same person twice. The agency gives each consultant a personal DDI from its existing range, printed in their email signature and on job adverts. Unanswered calls after 20 seconds go to the consultant’s mobile app, and only general enquiries still use the main number.

What to check

  • Range size: reserve enough numbers for future hires so new staff stay in the same block.
  • Outbound presentation: make sure each user presents their own DDI when they should, rather than the main number.
  • Porting: when you switch provider, include the whole range in the number porting request.

DIDs at Universal Telecom

Our numbers work as DIDs on the virtual PBX and on your own PBX connected through our SIP trunk. For direct numbers in other countries, see global numbering.

Frequently asked questions

Is a DID the same as a DDI?

Yes. DID (direct inward dialling) is the term used in North America and by most VoIP platforms; DDI (direct dial inward) is the usual British name. Both describe a public number routed straight to an extension or group.

Does every DID need its own phone line?

No. All DIDs share the same connection to the carrier, whether a SIP trunk or a cloud PBX. The dialled number tells the phone system where to send the call, so you can have far more DIDs than simultaneous call channels.

Can I keep my DDI range when I change provider?

Usually yes. Geographic DDI ranges can be ported in the UK and Ireland. List every number in the range on the port request, because numbers left off may be lost when the old service is cancelled.

Related terms

  • 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.

  • PBX

    A PBX (private branch exchange) is a business's own telephone switch. It connects internal extensions to each other and to the public network, and runs features such as IVR menus, queues and voicemail.

  • Cloud PBX

    A cloud PBX is a business phone system run by a provider in its data centres. Extensions, IVR menus, queues and call routing are managed from a web portal, with no PBX hardware on site.

  • 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.

  • Number porting

    Number porting is moving an existing phone number from one provider to another so the business keeps the same number. The new provider runs the process; the old service ends when the port completes.

  • IVR (interactive voice response)

    IVR (interactive voice response) is the automated phone menu that greets callers and routes them based on the keys they press, such as “press 1 for sales, press 2 for support”.

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