DID number
DID numbers: a direct line to a person or an AI agent.
A DID number, short for direct inward dialing, is a phone number that reaches one specific destination without an operator or a menu in between. The destination used to be an extension behind an office switchboard. Today it can be a desk phone, a team, a softphone or an AI voice agent.
To the caller a DID is an ordinary phone number. The difference is on your side: the carrier delivers each number’s calls over your connection, and your system routes them by the number dialled. VocalStack Voice provisions local and toll-free DIDs by API and points them straight at an agent.
Instant DIDs by API · Local & toll-free · Voice & SMS · Numbers in 50+ countries · SIP · WebRTC · PSTN
- System00:00
Inbound call to +44 20 7946 0321 · DID mapped to the London bookings agent
- AI agent00:01
Good afternoon, Calder Hotels London. I am the AI booking assistant. How can I help?
- Caller00:05
Hi, I would like a double room for two nights from the 14th.
- AI agent00:09
I have a double for the 14th and 15th at our King’s Cross hotel. Shall I hold it in your name?
- Caller00:14
Yes, please. And can I talk to someone about a group booking next month?
- System00:18
Room held · warm transfer to group sales · transcript attached
Direct inward dialing, in plain terms.
The standard telecom definition of DID is narrow: a service feature that lets calls into a PBX reach a specific extension without anyone putting them through. These are the terms that come up around it.
| What it means | |
|---|---|
| DID (direct inward dialing) | A service feature that allows inward-directed calls to a PBX to reach a specific PBX extension without human intervention. In practice, a phone number that routes straight to one destination. |
| DID number | An ordinary local or toll-free phone number that a carrier delivers to your phone system or platform, rather than to one phone line. |
| Extension | An internal line behind a PBX. Without a DID, a caller reaches it through a receptionist or a menu. |
| PBX | The private switch, in the office or in the cloud, that routes calls between your own lines and the public network. |
| SIP trunk | A connection that carries calls between your system and a carrier over the internet. Many DIDs can ride on one trunk. |
| E.164 | The ITU-T international public telecommunication numbering plan, which gives every DID its country code and international form. |
DID definition quoted from the ATIS Telecom Glossary, as of Nov 2026. E.164: ITU-T Recommendation, edition in force February 2026. · Source: ATIS Telecom Glossary, direct inward dialing
How a call to a DID reaches an AI voice agent.
The caller dials a normal number. Everything after that is routing, and each step is configuration rather than wiring.
- 01Caller dials the DIDFrom any phone, anywhere.
- 02Public networkHands the call to the carrier that holds the number.
- 03CarrierTwilio, Plivo, Exotel, Vobiz or a SIP trunk.
- 04Your platformLooks up the number dialled and picks the agent.
- 05AI voice agentAnswers in the business name for that number.
- 06Person, if neededWarm transfer with the transcript attached.
- One number, one destination
- Each DID can point at its own agent: a sales line, a support line, a line per branch or per client.
- Bring the numbers you have
- Import the DIDs on your own Twilio, Plivo, Exotel or Vobiz account and assign a published agent to answer them.
- Or provision new ones
- VocalStack Voice searches, buys and activates a DID with a single API request.
- Numbers held elsewhere
- If a contact-centre platform already holds your numbers, it can stream call audio to the agent over a WebSocket instead.
Local DID, toll-free number or extension.
All three can lead to an AI voice agent. They differ in who pays for the call and in how callers read the number.
| Local DID | Toll-free number | Extension without a DID | |
|---|---|---|---|
| Who pays for the call | The caller, like any call to that area | The number’s owner pays the toll charges | The caller, to reach the main number |
| How callers see it | A number in a city or region | A national number that is free to call | Not visible; reached through the main line |
| Straight to one destination | Yes | Yes | Only through a receptionist or a menu |
| Good for | Local presence, a line per branch or client, outbound caller ID | National sales and support lines | Calls between colleagues |
| With an AI voice agent | Each number mapped to its own agent | One national line answered by an agent | The agent can transfer calls to it |
Toll-free, as defined in US rules (47 CFR § 52.101): a number for which the toll charges for completed calls are paid by the toll free subscriber. Charges for calling toll-free numbers from mobiles vary by country. As of Nov 2026. · Source: LII, 47 CFR § 52.101
DID numbers on VocalStack Voice.
VocalStack Voice is the telephony layer under VocalLabs agents. A number is one API request away, and it arrives already connected to the AI.
Point a DID at an AI voice agent.
- 01
Get the number
Provision a local or toll-free DID through VocalStack Voice, or import the DIDs already on your Twilio, Plivo, Exotel or Vobiz account.
- 02
Publish the agent
Build the agent that should answer: its prompt, models, FAQs, tools and transfer rules. Test it in the browser first.
- 03
Map the number to the agent
Assign the published agent to the DID for inbound calls. Different numbers can point at different agents.
- 04
Call it yourself
Dial the number, try the awkward cases and read the transcript before the number goes on your website or your adverts.
What teams use DIDs for with AI voice agents.
- 01Local
A line per branch
Each location gets its own local number and an agent that knows that branch’s hours and address.
- 02Tracking
A line per campaign
A separate number on each advert or mailer, so every call shows which campaign it came from.
- 03White-label
A line per client
Agencies give every client their own numbers and agent, isolated from the rest.
- 04Outbound
Local caller ID
Outbound calls show a number from the region being called, and a callback reaches the same agent.
- 05Toll-free
A national support line
A toll-free number answered by an agent around the clock, with transfers to your team.
- 06Routing
Departments without a menu
Sales, billing and support each on their own DID, so callers never hear “press 1”.
Questions buyers ask first.
A number in. An agent on the line.
Tell us which countries you need numbers in, local or toll-free, and what should answer them. We will provision the DIDs, map them to your agents and let you call them before launch.
