If you're routing inbound calls to a PBX, a team or a voice AI agent, you'll be buying DIDs. This post follows a dialled number hop by hop until it lands on your system over SIP, and helps you work out how many DIDs you need.
DID stands for Direct Inward Dialing. It means a carrier delivers a phone number's calls over a shared trunk, along with the digits the caller dialled, so your system can send the call straight to one person, team, or application. Hundreds of DID numbers can share one trunk.
Europe and Oceania often say DDI, for direct dial-in. If you searched for "DI number" in a telecom context, you probably mean the same thing. This guide follows the dialled number hop by hop, and helps you decide how many DIDs you need.
Note: Key takeaways
- DID describes delivery, not a kind of number. A local, toll-free, or virtual number can be a DID as well.
- The dialled digits are the routing key. They reach you inside a SIP message or as a field in webhooks (callbacks).
- Numbers and channels are separate counts. In our worked example, 500 DIDs need only 25 channels.
- Buy numbers for identity, not capacity. Add one when a person, campaign, tenant, or agent needs its own address.
- Direction and portability vary. A number may be inbound, outbound, or both, and keeping it when you change provider depends on the country and the number type.
What is the full form of a DID number?
The full form is Direct Inward Dialing number. The name describes its first job, which was letting an outside caller dial inward to one extension without passing through an operator.
The service works in two parts. The telephone company allocates a range of numbers and connects them to your PBX over one or more trunks. It then signals which number was dialled, and the PBX uses that to pick the extension.
Wikipedia credits AT&T with developing the feature in the US in the 1960s, modelled on an earlier German service. That entry carries a needs-citations notice, so treat the date as commonly repeated rather than settled.
A DID number example
Assume a company rents a block of 100 numbers, from +1 415 XXXX 1000 to +1 415 XXXX 1099, and has 30 channels.
- A caller dials +1 415 XXXX 1042. The company's system reads the digits and rings the collections team.
- Another caller dials +1 415 XXXX 1043. The same trunk delivers that call to a voice AI agent.
- Up to 30 calls can run at once, spread across any of the 100 numbers.
The same pattern works with a London +44 number, a Bengaluru +91 number, or any other range your carrier assigns.
To each caller, this looks like an ordinary phone number. Nothing about a DID is visible from outside.
How does the dialled number reach your system?
Everything a DID does depends on one piece of data surviving the trip: the number the caller dialled. Follow it through a call.
The form of those digits depends on how the call is delivered:
| Delivery | How the digits arrive | What you match on |
|---|---|---|
| Analog or PRI trunk | Through DNIS, in a prearranged and often partial format such as the last four digits | A short extension string |
| SIP trunk | Inside SIP URIs. RFC 3261 says the initial Request-URI should match the To field | The full number, after you normalise its format |
| Application webhook | As the dialed-number parameter sent to your answer URL | The full number in your code |
| SIP registration | The provider sends the call straight to a registered phone or soft-phone | Nothing, because the binding already chose the device |
What is a SIP DID?
A SIP DID is a DID delivered over SIP rather than over a PRI or analog circuit. Three words get mixed up here, and each counts something different:
- The DID is the address callers dial.
- The SIP trunk is the configured connection that delivers calls for many addresses.
- The channel is one call in progress on that connection.
On Vobiz, the inbound side of this is an inbound trunk. Each trunk forwards its numbers' calls to a primary origination URI, the host, port, and transport of your PBX or AI platform, and you can keep a second URI ready as a backup.
Two format traps in SIP routing
Number format. A global number under E.164 has at most fifteen digits, starting with a country code. Your PBX may hold rules in another form, such as a four-digit extension. Convert the incoming number to one form before you match it. Our E.164 guide covers the details.
Forwarded calls. RFC 3261 says the To field identifies the original recipient, which may not be the system now handling the request. Decide which field your dial plan trusts, then test it with a forwarded call.
When a DID misbehaves
| What you see | Likely cause | Check first |
|---|---|---|
| Number bought, but inbound calls never arrive | The number is owned but not yet routed | Its application or trunk link |
| Call reaches your PBX, then fails or lands on a default route | The dial plan expects a different digit format | Logged digits against the rule |
| A forwarded call reaches the wrong team or tenant | Your rule reads a field that changed in transit | The Request-URI against To |
| Callers hear busy while the number looks healthy | All channels are in use, regardless of how many DIDs you hold | Live concurrency |
| Outbound call rejected just after you bought the number | A newly bought number may still be registering | The time since purchase |
A cloud DID has no handset to pick up, so the first test is awkward. On Vobiz, the WebRTC Playground turns a browser tab into a softphone, so you can confirm a new number rings before you point it at an application or a trunk.
How does a DID differ from virtual, toll-free, and mobile numbers?
These terms overlap because each answers a different question. One number can fit several of them at once.
| Term | The question it answers | Relationship to DID |
|---|---|---|
| DID | How does the call reach your system? | Not applicable |
| Virtual number | Is it free of a physical line or SIM? | Most virtual numbers used for inbound calls are also DIDs |
| Toll-free number | Who pays? The called party typically bears the cost | Can be delivered as a DID |
| Mobile-format number | Which number range does the caller see? | Can be delivered as a DID |
| Extension | Which internal phone rings? | Not a public number, though a DID can map to one |
| Direct outward dialing (DOD) | How do internal users call out? | The outbound counterpart, often combined with DID |
So is a DID different from a regular phone number? From the caller's side, no. The difference is on your side, where a number points to a rule you control rather than to a fixed line.
Do you need DID numbers, and how many?
Start from what the number has to identify. A shared number with an IVR is often enough, and each extra DID should earn its place.
| Your situation | What to use | Why |
|---|---|---|
| One brand line, and callers choose a department | One number and an IVR | Callers decide, so you don't need separate addresses |
| Key people or accounts need a direct line | One DID per person or account | The caller skips the menu |
| You want to measure marketing sources | One DID per campaign | The dialled number shows which source drove the call |
| You run a multi-tenant product | One or more DIDs per tenant | Routing and records stay separate. On Vobiz, you can assign a DID to a sub-account |
| You run several voice AI agents | One DID per agent or flow | Each number points to its own agent and shows up in call records |
| You serve many cities | One local DID per city | Callers see a local code, and all calls reach the same system |
| Your outbound answer rate is falling | Fix pacing and your list, not the number | Rotating numbers to dodge flags gets penalised |
Worked example: numbers by identity, channels by load
Assume you hold 500 DIDs, one for each salesperson and campaign. In the busiest hour, you take 240 calls that last 4 minutes each.
- Load: 240 × 4 ÷ 60 = 16 erlangs.
- Channels: Erlang B gives 25 channels for 0.93% blocking, which meets a 1% target.
- Bandwidth: At about 87 kbps per G.711 call, a figure used by TRAI, that's about 2.2 Mbps.
Those are our own calculations. If the same traffic came through 5 numbers, you'd still need 25 channels. Our SIP trunking guide explains the formula, and the CPS and concurrency calculator models outbound campaigns.
What should you ask a DID provider?
Whichever provider you consider, these questions separate one offer from another:
| Ask | Why it matters |
|---|---|
| Which countries and series can you supply, and in which direction? | "DID" alone doesn't tell you whether a number takes inbound calls |
| What verification comes first? | KYC requirements set your real lead time |
| What can a number route to? | You may need a webhook now and a SIP trunk later |
| How are channels sold and limited? | Your number count doesn't set your capacity |
| Can I see each number's answer rate and spam status? | You want warning before a number burns out |
| What are the minimum term, release terms, and cool-off rules? | They decide how freely you can change your number estate |
How Vobiz handles DID numbers
Vobiz treats a number as a routable resource that you manage in the console or over the Phone Numbers API.
- Finding numbers. You browse inventory by country and digit pattern, and each number lists its capabilities and fees.
- Buying. A purchase debits the setup fee and first monthly fee from your balance, and a step-by-step guide walks through it. Use your own Auth ID and Auth Token, and confirm the number first. Current rates are on the Vobiz pricing page.
- Routing. A number routes through an application or a SIP trunk, not both. An application fetches VobizXML from your answer URL. A trunk forwards the call to an origination URI on your PBX or AI platform.
- India numbers. If you're in India, KYC comes before you rent a number. The provisioning guide covers 140 and 160 DIDs, the 92 series guide covers mobile-format numbers, and the calling regulations set which series you can use.
- Tenants. You can assign a DID to a sub-account. Moving a recently used DID back to the parent pool has a 15-day cool-off, which keeps a recently active number from reaching another customer too soon.
- Reputation. The Number Health API returns each number's status, spam flag, answer rate, and call metrics. The number health guide covers warm-up and pacing.
- Moving to Vobiz. You buy a new Vobiz number from inventory, point your call flows at it, and keep your old number live during the switch.
- Reach. Voice AI platforms such as LiveKit and Vapi connect to your numbers over SIP.
Frequently asked questions
What is the full form of a DID number?
DID stands for Direct Inward Dialing. It means a phone number's calls arrive over a shared trunk with the dialled digits attached, so a call can go straight to one person, team, or application. In Europe and Oceania, the same service is often called DDI, for direct dial-in.
What is an example of a DID number?
A company rents 100 numbers from +1 415 XXXX 1000 to +1 415 XXXX 1099 on one trunk. A caller dials the number ending 1042, and the system routes that call to collections. To the caller, it looks like any ordinary phone number. The same idea applies to a +44 or +91 range, or any other block your carrier assigns.
How does a DID differ from a SIP trunk?
A DID is an address that callers dial. A SIP trunk is the configured connection that delivers calls for many DIDs to your system. One trunk can carry many numbers, and the channel count, not the number count, limits how many calls run at once.
Can a DID number make outbound calls?
Often, yes. The name describes inbound delivery, but a provider can also let the same number act as your caller ID. Direction still varies by number series, so always ask which directions a number supports. If you're in India, 140 and 160 series numbers are outbound only, so you pair them with a standard DID for inbound calls.
How is a DID different from a virtual number?
The terms answer different questions. Virtual means the number isn't tied to a physical line or SIM. DID means the call and its dialled digits reach your system so you can route them. Most virtual numbers that take inbound calls are also DIDs.
Can I keep my DID number when I change provider?
It depends on the country and the number type. Portability rules differ by country, and some number types are harder to move than others. If you're in India, mobile numbers can be ported, while fixed-line and toll-free portability is more limited. Ask your provider, and plan to run old and new numbers side by side while callers switch.
Further reading on Vobiz
- Inbound trunks · What is a landline number?
- UCC management · 140, 160, and 92 series FAQ
- How VobizXML works · Number utilization guide
Sources
Standards
- ITU-T, Recommendation E.164: The international public telecommunication numbering plan.
- IETF, RFC 3261: SIP: Session Initiation Protocol, June 2002.
Reference
- TRAI, Recommendations on Revision of National Numbering Plan, 6 February 2025.
- Wikipedia, "Direct inward dial".
- Wikipedia, "E.164".
- Wikipedia, "Local number portability".
If a DID is going to ring a voice AI agent, Vobiz provides the numbers and SIP trunking to connect it.
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