This guide is for developers and operations teams putting an AI voice agent — software that speaks and listens on a call — onto the real phone network. It assumes you already have, or are choosing, an agent platform, and focuses on the part that platform does not give you: the number and the connection that let the agent place and take calls. A DID (Direct Inward Dialing) number, also called a virtual number, is a public phone number provisioned online and delivered over SIP (Session Initiation Protocol — the signalling standard used to set up and tear down internet calls). Instead of being wired to a fixed line, the number is software-defined: you point it at the endpoint your agent listens on.
We cover why an agent needs a number at all, how a call actually reaches it, the ways to connect (including bring-your-own-carrier to hosted platforms), how local numbers in many countries change answer rates, and the honest limitations to plan around. Terms are defined inline as they appear. If you would rather see the commercial summary first, the phone numbers & SIP for AI voice agents page is the short version of this guide.
On this page
- What is a virtual number for an AI agent?
- Why does an AI voice agent need a number?
- How does a call reach an AI agent?
- Ways to connect a number to an agent
- Local numbers in many countries
- Inbound and outbound calls
- What to check before you connect
- Which AI platforms it works with
- Where to go next
- FAQ
What is a virtual number for an AI agent?
A virtual number for an AI agent is an ordinary DID — a public phone number that anyone can dial — that you route to the place your agent runs instead of to a desk phone or PBX. Nothing about the number is AI-specific; what makes it work for an agent is where you point it and how the call is carried. Because the number is delivered over SIP, the destination is just a SIP endpoint, and an AI platform that speaks SIP is a valid endpoint like any other. For the underlying definitions, see what is a DID number and what is a virtual phone number.
Why does an AI voice agent need a number?
An agent platform gives you speech, reasoning and a voice, but not a place on the phone network. A number is what lets people dial the agent, and a SIP connection is what carries their audio to it and the agent's audio back. Without both, the agent can only run inside an app, a browser widget or a test console — never on a normal phone call. Many platforms bundle a starter number to get you going, but those are frequently limited to one country, so an agent meant to serve customers in several markets needs numbers you control across those markets. That is the gap a virtual-number provider fills: the number, and the trunk that connects it to your agent.
How does a call reach an AI agent?
A call reaches an AI agent by the same path as any DID call, with the agent as the final destination. When someone dials your number, the call crosses the PSTN (Public Switched Telephone Network — the traditional interconnected phone network) to the carrier that holds the number range. That carrier hands the call to your number provider, which converts it into a SIP INVITE — the SIP request that opens a new call and carries the dialled number — and sends it to the endpoint you configured. For an AI agent that endpoint is your platform's SIP address, and the platform attaches the call to the right agent.
From there it is signalling plus media: SIP sets the call up and tears it down, and a separate audio stream carries the voice both ways so the agent can hear the caller and speak back. Voice is typically carried with standard codecs such as G.711, which most platforms accept. Because the last leg is SIP, you can change where a number points at any time without changing the number. For the detailed call path, including trunks and authentication, read how DID numbers work with SIP trunking.
Ways to connect a number to an agent
There are two things to decide: where the agent runs, and how the trunk that feeds it is authorised. Most teams run their agent on a hosted AI platform and bring their own carrier to it — the pattern usually called BYOC (bring your own carrier) or bring-your-own-number. Others run their own softswitch or self-hosted stack and connect the trunk directly. Either way, a SIP trunk is authorised in one of two standard ways, and the table sets them side by side.
| Connection | What it is | Good fit when | Watch for |
|---|---|---|---|
| IP-allowlist interconnect | The trunk trusts calls to and from a fixed list of IP addresses — no login, just authorised addresses. | You have stable, static IPs (a hosted platform's SIP region, or your own server) and want the simplest high-volume path. | You must keep the allowlisted addresses current; calls from unlisted addresses are refused by design. |
| Registered SIP credentials | The endpoint registers with a username and password, so the trunk works from any address that authenticates. | Your agent or gateway does not have a fixed IP, or you prefer credential-based auth. | Credentials must be stored securely and rotated; treat them like any other secret. |
| BYOC to a hosted platform | You point a SIP trunk at an AI platform that supports bring-your-own-carrier and map your DID to your agent. | You use Vapi, Retell, ElevenLabs, OpenAI or LiveKit and want your own numbers instead of bundled ones. | BYOC support and exact steps differ per platform; follow the platform's own SIP / BYOC documentation. |
Whichever you choose, the credentials or allowlisted addresses are provisioned privately to your account — they are never published on this site. For the trunk-and-endpoint path in general, see SIP trunking.
Local numbers in many countries
A single foreign number hurts answer rates: people are far less likely to pick up an unknown international caller ID. Giving your agent a local number in each market it serves — a geographic city number, a national number, a toll-free number, or a mobile-range number, depending on the country — makes the call read as in-country, which lifts pickup and trust. Because DIDs are software-defined and delivered over SIP, one account can hold numbers across many countries and route them all to the same agent. Local, national, toll-free and mobile-range numbers are available across 100+ countries, with the exact types and any verification shown per market. Browse per-country availability in the coverage table.
Inbound and outbound calls
An agent usually needs both directions, and a virtual number handles both over the same trunk. On inbound, your DID originates the call into the agent: someone dials the number, the call arrives as a SIP INVITE, and the platform connects it to the agent. On outbound, the agent places the call over the trunk and presents a caller ID you are authorised to use — often the same local number, so a callback reaches the same agent. Outbound calling is governed by the rules of the destination country and by acceptable-use and anti-fraud controls, so campaigns and dialling patterns should follow both. See the acceptable use policy for the boundaries.
What to check before you connect
A few checks up front prevent the common surprises. None are unique to AI agents, but each matters more when an automated system is placing or taking calls at volume:
- Codec compatibility. Confirm your platform accepts the codec the trunk offers — standard codecs such as G.711 are widely supported. A codec mismatch shows up as one-way or no audio even when signalling looks fine.
- Concurrency and channels. A channel is one concurrent call path. An agent that answers many calls at once needs enough channels; size concurrency to your busiest expected moment, not your average.
- SMS support varies. Not every number sends or receives SMS, and messaging rules differ by country and number type. If your agent flow needs texts — for confirmations, or OTP (one-time passcode) delivery — confirm SMS capability on the specific number before you rely on it.
- Per-country verification. Many markets attach KYC (Know Your Customer — identity and eligibility checks) requirements to their number ranges, from none to ID to proof of address to evidence of local presence. Where documents are reviewed, activation is not instant, so build that lead time into any launch. See regulations and KYC.
- Porting is not instant. Bringing an existing number in, or taking one out, is governed by industry rules and takes days to weeks. Confirm portability before you schedule a cutover; details are on the number porting page.
- Not for emergency calling. A virtual number has no fixed physical location, so it cannot reliably tell an emergency service where a caller is. Do not rely on an agent's number to reach emergency services, and keep a separate, location-based means of doing so.
Which AI platforms it works with
Any platform that speaks SIP can take a call from a virtual number. In practice that includes hosted voice-AI platforms such as Vapi, Retell, ElevenLabs, OpenAI's Realtime API and LiveKit, as well as self-hosted agents and SIP-capable softswitches like FreeSWITCH, Asterisk or 3CX. The connection method is the same standard SIP trunk described above; only the platform's configuration screen differs. Because BYOC support and setup steps vary, always follow the platform's own SIP or BYOC documentation for the exact fields.
Temptera is an independent carrier and is not affiliated with, endorsed by, or sponsored by any of the AI platforms named above; the names are used only to describe interoperability.