Direct inward dialing (DID) is a phone service that lets a business offer many direct phone numbers over a shared set of lines or a cloud phone service, with each number ringing straight through to a specific person, department, queue or device instead of a main switchboard. Originally DID described a feature of the trunks connecting a carrier to a company’s private branch exchange (PBX). Today “a DID” usually just means one of those direct numbers, delivered over SIP trunking or a hosted PBX or other cloud phone service.
At a glance
- A DID is a regular local phone number that routes directly to a user, group or device inside your phone system.
- Many DIDs share the same trunks or cloud service, so you don’t pay for a physical line per number.
- Numbers are usually rented from a carrier, often in blocks, and can generally be ported between providers.
- Numbers and calling capacity are separate: you might have hundreds of DIDs and far fewer simultaneous call paths.
- DIDs underpin direct lines for staff, department and fax numbers, campaign tracking and local presence in other areas.
What problem it solves
Before DID, outside callers reached a company through a main number and an operator or menu transferred them. That works for small offices, but it slows callers down, ties up reception and makes it hard for employees to give customers a direct number. Giving each person a separate physical line would be expensive and wasteful, since many lines sit idle much of the time.
DID separates numbers from lines. The business holds a set of numbers, the carrier delivers calls to any of them over the same trunks or service, and the phone system uses the number dialed to ring the right destination. Employees get a direct number that follows them to whichever device they use, and the company only pays for the calling capacity it needs.
How it works
Getting numbers. The business rents numbers from its carrier or phone service provider, either new numbers in the area codes it wants or existing numbers ported in through local number portability. Availability of specific area codes, number ranges and international numbers varies by provider and country.
Delivering calls. When someone dials one of the numbers, the carrier sends the call over the business’s trunk, whether a legacy PRI circuit, a SIP trunk or a cloud phone platform, and includes the dialed number in the call signaling. That dialed-number information is the same thing contact centers use as DNIS.
Routing. The PBX or cloud phone system maps each DID to a destination: a user’s extension, a hunt group or queue, an auto attendant, a fax endpoint or voicemail. With VoIP and cloud systems, the destination can be a desk phone, a softphone on a laptop or a mobile app.
Outbound identity. DIDs are about inbound calls, but many systems also show a user’s DID as the outbound caller ID, so customers can call back directly. The name shown to recipients is set separately through caller ID name (CNAM) where the receiving carrier supports it.
When it matters for buyers
- When replacing a PBX or PRI. Every number has to be moved and re-mapped. A complete number inventory avoids losing numbers customers still call.
- When opening a new office or adding remote staff. Local numbers in new area codes and direct lines for remote employees are usually quick to add on a cloud service.
- When comparing quotes. Providers charge differently for numbers: per number per month, in blocks, or bundled with user licenses, with separate charges for porting or special number types.
- When securing the phone system. Fraud usually targets the phone system’s outbound calling rather than the numbers themselves, but published numbers lead anyone who calls, including fraudsters, into voicemail and menus, so voicemail PINs, transfer rules and outbound dialing permissions need locking down.
Questions to ask vendors
- How are DIDs priced: per number, in blocks or included with user licenses?
- Which area codes and countries can you provide numbers in, and how quickly?
- What does porting cost, how long does it typically take and who manages the request?
- How many simultaneous calls does our plan support, separately from how many numbers we hold?
- Can we route each number ourselves, including after-hours and failover rules?
- If we leave, will you release our numbers promptly for porting to another provider?
- Do you set outbound caller ID and caller name for each number, and is there a charge?
How it differs from a SIP trunk
A SIP trunk is the connection that carries calls between your phone system and the public phone network, sized by how many calls it can carry at once. DIDs are the phone numbers that ride on it. Buyers often see both on the same quote: a number of call paths or a calling plan, and a number of DIDs. The two scale independently, so a company with 200 direct numbers might only need enough paths for its busiest hour. A toll-free number is another kind of number that can be delivered over the same trunk; unlike a DID, its owner pays for incoming calls. For trunk options and pricing, see our SIP trunking page.
Problems this helps with
- Outside callers have to go through reception to reach anyone
- Need direct numbers for staff without paying for a separate line each
- Moving to a cloud phone system and need to keep our existing numbers
