Phone Calling

Connect Vision Agents to PSTN phone calls. A telephony provider handles the phone network; your FastAPI server receives webhooks, streams audio over WebSocket, and bridges it into a Stream call where your agent runs.

Vision Agents uses Stream Video for real-time WebRTC transport by default. External WebRTC transports are supported as well. Most AI providers offer free tiers to get started.

Choose a provider

Twilio

Media Streams, TwiML, built-in webhook helpers — default in our examples

Telnyx

Call Control API, bidirectional media streaming

How phone agents work

Every phone integration follows the same pattern, regardless of provider:

1. Webhook — the provider notifies your server of an incoming call (or you initiate an outbound call via REST API)
2. Register — create a call entry in a provider registry with a secret token
3. Stream URL — tell the provider to open a WebSocket media stream to your server
4. Bridge — attach_phone_to_call connects phone audio ↔ a Stream call where your agent listens and responds

Shared building blocks

Both telephony plugins expose the same core primitives:

Building block	Role
Call registry	Tracks active calls, assigns tokens, optional async prepare to warm up the agent
Media stream	WebSocket handler that receives/sends phone audio
attach_phone_to_call	Bridges provider audio ↔ Stream WebRTC participant
Tokenized media URL	Provider connects to wss://<host>/<provider>/media/{call_id}/{token}

Provider-specific differences:

	Twilio	Telnyx
Answer mechanism	Return TwiML with <Stream>	Call Control Answer/Dial API with stream_url
Webhook auth	verify_twilio_signature (built-in)	Ed25519 verification (see plugin examples)
Audio encoding	mulaw 8 kHz	PCMU/PCMA 8 kHz or L16 16 kHz

Prerequisites

Before starting any phone example:

• Stream API key and secret
• A telephony provider account with at least one phone number
• ngrok (or another HTTPS tunnel) for local development
• A public webhook URL pointing at your FastAPI server

Prefer managed hosting? Stream Voice AI runs production voice agents on Stream’s global edge, phone numbers, web and mobile clients, co-located STT/LLM/TTS, and built-in observability. Join the waitlist for early access.

Learning path

Follow this order:

1. Twilio Phone Agent — get your first inbound and outbound call working
2. Phone Support Agent (RAG) — add knowledge retrieval on top
3. Twilio or Telnyx integration pages — API reference when building your own server

Twilio Phone Agent

Step-by-step phone tutorial

Phone Support Agent (RAG)

Add knowledge retrieval to calls

Quick orientation

1. Expose your local server

   Run ngrok http 8000 and note the HTTPS hostname for NGROK_URL.

2. Point webhooks at your server

   Configure your provider to send call events to your FastAPI endpoints (e.g. https://<NGROK_URL>/twilio/voice for Twilio).

3. Run an example and call your number

   Follow the Twilio Phone Agent tutorial for full commands. Dial your Twilio number to verify end-to-end audio.

Next Steps

Twilio Integration

Plugin API reference

Telnyx Integration

Alternative provider

RAG for Agents

Knowledge retrieval backends

Stream Video RTC

Default edge transport

Build a Voice Agent

Voice agent fundamentals