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AI Dialer

Glossary · Numbering

What is DID (direct inward dialing)?

  • Also called: direct inward dialing, ddi, direct dial number

Reviewed by Sujan ThapaliyaLast updated

Definition

A DID is an external phone number that rings a specific person or team directly, without going through a switchboard or an extension. Known as DDI outside North America, it is how a business gives every seat its own number without a physical line each.
The three STIR/SHAKEN attestation levels, which underpin how did (direct inward dialing) behaves in practice: A asserts both that the carrier knows the customer and that the customer may use the calling number, B asserts the customer only, and C asserts neither. A shared number pool can reach B at best, which is why number ownership keeps recurring in these definitions.
Attestation is the backdrop to most caller-ID and deliverability terms, did (direct inward dialing) included.

In practice

How did (direct inward dialing) actually works

The economics changed with VoIP. DIDs used to require blocks of numbers mapped onto physical lines; now a number is a routing rule, so a business can hold as many as it needs and point each wherever it likes.

For outbound calling, DIDs are what make caller identity honest: the number presented is a real number that rings back to the person who called.

The common mistake is buying DIDs without deciding what answers them out of hours. A direct number that rings unanswered at 6pm is worse than a switchboard that routes to a queue.

DIDs also change the shape of inbound traffic in a way worth planning for. Publishing direct numbers removes the switchboard's filtering, so individuals receive calls that reception used to triage — including the sales calls. Teams that hand out DIDs without an out-of-hours or busy path usually regret it within a month.

Worked example

Forty staff on one main number with extensions means every caller passes reception. Forty DIDs means each person is reachable directly — and each needs an out-of-hours destination, or the direct number rings out at 6pm.

DID (direct inward dialing): common questions

What is a DID number?
An external number that rings a specific person or team directly, rather than reaching a main switchboard first. DDI is the same thing under a different name.
How is a DID different from an extension?
An extension is internal and only usable once a caller has reached the main number. A DID is a full public number that reaches the same handset from anywhere.
Do DIDs cost more than a main number?
Each carries a monthly rental, so cost scales with how many you hold. On a usage-priced platform that rental is small enough that the deciding factor is routing rather than budget.

Sources

  1. Combating Spoofed Robocalls with Caller ID Authentication — Federal Communications CommissionThe STIR/SHAKEN framework, the attestation levels carriers sign calls with, and the mandate requiring providers to authenticate caller ID.
  2. 47 U.S.C. § 227 — Restrictions on the use of telephone equipment — Cornell Legal Information InstituteThe Telephone Consumer Protection Act itself — the consent requirements, calling-hours limits, and private right of action.
  3. ITU-T Recommendation E.164 — The international public telecommunication numbering plan — International Telecommunication UnionThe international number format, the 15-digit maximum, and how country codes and national numbers compose.

See did (direct inward dialing) in the product

A toll-free number is one the called business pays for rather than the caller. In North America they use the prefixes 800, 833, 844, 855, 866, 877 and 888, which are functionally identical — the newer ones exist only because the older ones ran out. They signal national reach; a local number signals proximity.

Explore toll-free numbersGet early access