AI voice agent platforms such as ElevenLabs, Vapi, LiveKit and Retell can connect to the phone network through your own SIP trunk: you point the trunk at the platform's SIP address for inbound calls, give the platform your trunk's address and credentials for outbound calls, and agree on codecs and security. A good trunk keeps latency low, delivers your caller ID and accepts AI calling as a declared traffic type.
Why use a SIP trunk for AI voice agents?
Each platform offers numbers through built-in telephony partners, which is the fastest way to start. A bring-your-own SIP trunk is worth it when you need:
- Numbers or termination in countries the built-in option does not cover.
- Lower cost at volume, by buying minutes at wholesale rates.
- Your existing numbers and caller IDs, already registered and reputable.
- The same carrier for AI agents and human agents, so routing and billing stay in one place.
If SIP trunks are new to you, start with what is SIP trunking.
How do you connect a SIP trunk to an AI voice platform?
- Inbound: create the number on your trunk and route it to the platform's SIP URI. On the platform, import the number and attach it to an agent.
- Outbound: on the platform, enter your trunk's SIP address and either username and password (digest authentication) or allowlist the platform's IP addresses on your trunk.
- Transport and security: choose UDP, TCP or TLS as both sides support. ElevenLabs, for example, documents TLS transport and media encryption, and asks you to allowlist its SIP server IP range.
- Codecs: offer G.711 first; it avoids transcoding. See VoIP codecs G.711 and G.729.
- Number format: send and expect numbers in E.164 format. See E.164 number format.
- Test inbound, outbound, DTMF, transfers to human agents and hang-up handling.
ElevenLabs, Vapi, LiveKit and Retell SIP trunking
All four document a bring-your-own SIP trunk setup, with the same building blocks: an inbound SIP endpoint on the platform, an outbound trunk definition with your provider's address and credentials, and number import. Field names and supported options change between releases, so follow each platform's current SIP trunking guide and ask your trunk provider for its connection details (SIP host, port, transport, authentication method, IP ranges).
What makes a good route for AI voice calls?
| Requirement | Why it matters for AI agents |
|---|---|
| Low latency and jitter | Every extra delay makes the agent sound slow and causes people to talk over it |
| No transcoding | Transcoding adds delay and degrades audio, which hurts speech recognition. See VoIP transcoding |
| CLI delivery | People answer recognizable numbers; non-CLI routes lower answer rates |
| Reliable DTMF | Agents often read keypad input for menus and verification |
| Enough channels and CPS | AI agents scale to many parallel calls quickly. See VoIP channels and CPS |
| Per second billing | AI calls are often short |
Choose a trunk point of presence close to the platform's region, and watch post dial delay and call quality and MOS in your CDRs.
Which rules apply to AI voice agent calls?
Inbound AI agents answering customers who call you raise few telecom issues. Outbound AI calls are regulated: in the US, the FCC ruled in February 2024 that AI-generated voices are "artificial" voices under the TCPA, so outbound AI calls need prior express consent, and written consent for telemarketing, plus do-not-call and calling hour rules. Rules differ by country, and some also require telling people they are talking to an AI. See are auto dialers legal. Declare AI calling to your route supplier: undeclared automated traffic is cut quickly.
Routes for AI voice platforms
VoIPMatcher sources VoIP termination for CPaaS and AI voice platforms from carriers that accept the traffic. Request a route for your AI voice agent with destinations, expected concurrent calls and use case.
Frequently asked questions
Do I need my own SIP trunk for an AI voice agent?
Not always. Most AI voice platforms can sell you numbers and calling through a built-in telephony partner. Bringing your own SIP trunk makes sense when you need numbers in more countries, lower per minute costs at volume, specific caller IDs, or routes your platform's partner does not offer.
Which codec is best for AI voice agents?
G.711 is the safest choice on phone networks: it is supported everywhere and avoids transcoding, which adds delay and can hurt speech recognition. Wideband codecs such as Opus sound better but work only where both sides and the route support them. Avoid routes that transcode to low bitrate codecs.
Can AI voice agents make outbound sales calls?
Technically yes, but the rules are strict. In the US, the FCC treats AI-generated voices as artificial voices under the TCPA, so outbound calls need prior express consent, and prior express written consent for telemarketing, plus do-not-call and calling hour rules. Other countries have their own rules, and route suppliers will ask about the use case.
How much latency is acceptable for an AI voice agent call?
For natural conversation, the total delay between the caller finishing a sentence and hearing the reply should stay as short as possible, ideally around a second or less. The network path is only part of it; speech recognition, the language model and voice synthesis add most of the delay. Choose routes and regions that keep network delay low.
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