Short Answer
Use WebRTC when the AI voice agent lives inside a browser or app, and use SIP/PSTN when callers dial phone numbers, PBXs, or contact-center routes. LiveKit and Daily are natural WebRTC and realtime infrastructure candidates. Telnyx and Twilio are natural SIP and programmable voice candidates. Managed platforms such as Vapi or Retell AI may hide complexity.
Quick Recommendation
| Voice-agent environment | Better starting path | Why |
|---|---|---|
| Browser or mobile app voice | WebRTC | Low-latency media and app context matter. |
| Customer calls a phone number | SIP/PSTN | Phone routing, carrier path, and transfers matter. |
| Existing PBX or contact center | SIP | Integration with current phone infrastructure matters. |
| Product has phone and in-app voice | Both | Different callers enter through different channels. |
| Team wants managed assistant layer | Vapi or Retell AI | Platform may hide some media/telephony complexity. |
What The Terms Mean
WebRTC is commonly used for real-time audio and video inside applications and browsers. SIP is commonly used to connect telephony systems, PBXs, carriers, and contact-center infrastructure. PSTN is the traditional public phone network path that many callers still use.
AI voice agents can sit on top of any of these paths, but the engineering and operating model changes.
Provider Fit
| Provider | Strong fit | Watch-outs |
|---|---|---|
| LiveKit | Real-time voice apps, agents, and app-integrated voice. | Requires engineering ownership. |
| Daily | Real-time media and voice/video infrastructure. | Pair with orchestration when building full phone agents. |
| Pipecat | Framework-level orchestration across transports. | Hosting and monitoring are buyer-owned. |
| Telnyx | SIP, carrier control, phone numbers, programmable voice, media streams. | More infrastructure responsibility. |
| Twilio | Programmable voice, call control, media streams, telephony ecosystem. | Voice-agent orchestration may still be custom. |
| Vapi and Retell AI | Managed phone-agent platforms. | Verify how much telephony and infrastructure control remains visible. |
Test Before You Build
Measure latency through the full path:
- Caller audio enters the system.
- Speech recognition starts and stabilizes.
- Model produces a response.
- Tool call runs, if needed.
- Text-to-speech starts.
- Audio reaches the caller.
- Transfer or recording event is available.
Then test interruption, noisy audio, transfer, and post-call transcript availability. The AI Voice Agent Latency Benchmark is the right companion protocol.
Pricing And Ownership Questions
Ask who owns phone numbers, SIP trunks, WebRTC rooms, media servers, recording, storage, model choices, voice providers, logs, alerts, and failover. The managed platform may simplify some of this; an infrastructure build may expose more control but demand more operations work.
Cost should include telephony, media, speech, model, text-to-speech, storage, engineering time, and incident response.
Source-Backed Evidence
Use infrastructure docs instead of vendor category labels. LiveKit documents realtime AI Agents in its Agents docs and should be reviewed when app voice or WebRTC media is central. Daily documents WebRTC and realtime media products on its developer site. Telnyx documents programmable voice and SIP-oriented infrastructure on its Voice API docs. Twilio documents live audio streaming with Media Streams. Vapi and Retell AI can simplify phone-agent workflows, but buyers should still ask where SIP, PSTN, recording, and transfer responsibility sits.
Exclusion Rules
Do not choose WebRTC just because it is modern if customers primarily dial phone numbers. Do not choose SIP/PSTN just because the company already has telephony if the product experience lives in-app. Do not use a managed platform to hide infrastructure questions when compliance, recording, failover, or carrier routing must be owned internally.
Related Reading
- Voice AI Infrastructure Stack
- Vapi vs LiveKit vs Pipecat
- LiveKit vs Daily for AI Voice Agents
- Twilio vs Telnyx for AI Voice Agents
- Telnyx vs Vapi
- AI Voice Agent Latency and Architecture Guide
Buyer FAQs
Is WebRTC or SIP better for AI voice agents?
WebRTC is usually better when the AI voice experience lives inside a browser or app. SIP is usually better when callers use phone numbers, PBXs, contact centers, or PSTN routes. Many production systems use both.
Can an AI voice agent use both WebRTC and SIP?
Yes. A product can use WebRTC for in-app voice and SIP or PSTN for phone callers. The important decision is where media, routing, recording, transfer, and observability are owned.
Why does telephony add latency to AI voice agents?
Telephony can add latency through PSTN routing, codecs, media streaming, speech recognition, model response time, text-to-speech, tool calls, and transfer logic. Buyers should measure the full path, not one vendor's model latency alone.
Do I need Twilio, Telnyx, Daily, LiveKit, or Vapi?
Use Twilio or Telnyx when phone infrastructure and call control are central, Daily or LiveKit when real-time media and app voice are central, and Vapi or Retell AI when a managed voice-agent platform should own more of the assistant workflow.