twilio-voice-conversation-relay

name	twilio-voice-conversation-relay
description	Build AI-powered voice agents using Twilio ConversationRelay. Handles real-time speech recognition (ASR), text-to-speech (TTS), and bidirectional audio streaming via WebSocket. Covers TwiML setup, WebSocket message types, LLM integration, streaming responses, and voice provider configuration. Use this skill to build voice bots, IVR replacements, or real-time AI voice assistants on Twilio calls.

Overview

ConversationRelay connects Twilio's telephony layer to your app via a persistent WebSocket. Twilio handles ASR (speech-to-text) and TTS (text-to-speech); your app receives transcripts, calls an LLM, and sends text back for playback.

Caller ←→ Twilio (ASR/TTS) ←→ WebSocket ←→ Your App ←→ LLM

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Prerequisites

• Upgraded Twilio account with ConversationRelay access (requires onboarding) — New to Twilio? See twilio-account-setup — Start onboarding at: Console > Voice > ConversationRelay — access is not instant
• A voice-capable Twilio phone number
• TWILIO_ACCOUNT_SID and TWILIO_AUTH_TOKEN — see twilio-iam-auth-setup
• WebSocket server reachable via wss:// (TLS required)
• An LLM integration (OpenAI, Anthropic, etc.)
• For placing calls: see twilio-voice-outbound-calls

Onboarding: Complete via Console > Voice > ConversationRelay > Onboarding. Select TTS/ASR providers:

• TTS: Deepgram, Amazon Polly, Google Cloud TTS, ElevenLabs
• ASR: Deepgram, Google Cloud STT

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Quickstart

Step 1 — Return TwiML pointing to your WebSocket server

Python (Flask)

from flask import Flask
from twilio.twiml.voice_response import VoiceResponse, Connect, ConversationRelay

app = Flask(__name__)

@app.route("/voice", methods=["POST"])
def voice():
    response = VoiceResponse()
    connect = Connect()
    connect.conversation_relay(
        url="wss://yourapp.com/ws/voice",
        welcome_greeting="Hello! How can I help you today?"
    )
    response.append(connect)
    return str(response)

Node.js (Express)

const { VoiceResponse } = require("twilio").twiml;

app.post("/voice", (req, res) => {
    const response = new VoiceResponse();
    const connect = response.connect();
    connect.conversationRelay({
        url: "wss://yourapp.com/ws/voice",
        welcomeGreeting: "Hello! How can I help you today?",
    });
    res.type("text/xml").send(response.toString());
});

Step 2 — Handle WebSocket events and respond with text

Python (websockets)

import asyncio, json, websockets

async def handle_call(websocket):
    async for message in websocket:
        event = json.loads(message)
        if event["type"] == "prompt":
            ai_response = await call_llm(event["voicePrompt"])
            await websocket.send(json.dumps({"type": "text", "token": ai_response, "last": True}))

async def main():
    async with websockets.serve(handle_call, "0.0.0.0", 8080):
        await asyncio.Future()

asyncio.run(main())

Node.js (ws)

const WebSocket = require("ws");
const wss = new WebSocket.Server({ port: 8080 });

wss.on("connection", (ws) => {
    ws.on("message", async (data) => {
        const event = JSON.parse(data);
        if (event.type === "prompt") {
            const aiResponse = await callLLM(event.voicePrompt);
            ws.send(JSON.stringify({ type: "text", token: aiResponse, last: true }));
        }
    });
});

Security: The voicePrompt field contains ASR-transcribed caller speech — it is untrusted external input. When passing to an LLM, isolate it as user input within a structured system prompt. Implement topic boundaries and output filtering to prevent the LLM from disclosing system instructions or speaking inappropriate content. ConversationRelay is a pure transport layer with no built-in content safety — any LLM output is spoken to the caller verbatim.

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Key Patterns

WebSocket Message Types

Received from Twilio:

Type	When	Key fields
connected	WebSocket opened	callSid, streamSid
prompt	User finished speaking	voicePrompt (transcript)
interrupt	User interrupted TTS	—
dtmf	User pressed keypad key	digit
error	An error occurred	description

Sent to Twilio:

Type	Purpose	Key fields
text	Send TTS response	token (text), last (bool)
interrupt	Stop current TTS	—
end	Hang up the call	reason

Stream LLM Responses Token-by-Token

Lower latency by streaming as the LLM generates output — Twilio starts speaking before the full response is ready.

Python

async for chunk in llm_stream:
    await websocket.send(json.dumps({"type": "text", "token": chunk, "last": False}))
await websocket.send(json.dumps({"type": "text", "token": "", "last": True}))

Node.js

for await (const chunk of llmStream) {
    ws.send(JSON.stringify({ type: "text", token: chunk, last: false }));
}
ws.send(JSON.stringify({ type: "text", token: "", last: true }));

Voice Configuration

Python

connect.conversation_relay(
    url="wss://yourapp.com/ws/voice",
    voice="en-US-Neural2-F",
    language="en-US",
    transcription_provider="deepgram",
    speech_model="nova-2-phonecall",
    interrupt_by_dtmf=True,
)

Node.js

connect.conversationRelay({
    url: "wss://yourapp.com/ws/voice",
    voice: "en-US-Neural2-F",
    language: "en-US",
    transcriptionProvider: "deepgram",
    speechModel: "nova-2-phonecall",
    interruptByDtmf: true,
});

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CANNOT

• No raw audio access — Text in, text out only. For raw audio, use <Connect><Stream> (Media Streams).
• Cannot mix with Media Streams — <Connect><Stream> and <Connect><ConversationRelay> are mutually exclusive on the same call. No error — one is silently ignored.
• No custom STT/TTS engines — Limited to Deepgram + Google (STT) and Deepgram + Amazon Polly + Google + ElevenLabs (TTS).
• No mid-session voice/provider changes — Voice and provider are set at TwiML time. Only language can be switched mid-session via WebSocket message.
• No WebSocket auto-reconnection — If the WebSocket drops, the call disconnects. Implement recovery via <Connect action> URL.
• Voice only — No SMS/messaging support. For omnichannel, use Conversation Orchestrator.
• No built-in memory or context — BYO conversation history and context management.
• No LLM integration — Pure transport layer. You bring your own LLM via the WebSocket server.
• No server-side recording via REST API — record:true on the Calls API is silently ignored. Must use <Start><Recording> before <Connect> in TwiML.
• Not PCI compliant with Voice Intelligence v2 — Do not enable intelligenceService in PCI workflows.
• ElevenLabs requires account enablement — Accounts without ElevenLabs access get error 64101. Voice IDs (not human-readable names) are required.
• Cannot use ConversationRelay without onboarding — Not available immediately on a new account
• Cannot use non-TLS WebSocket — Server must be reachable via wss:// (TLS required)
• Cannot stream audio without last: true — Twilio won't play audio until it sees a last: true token in the stream

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Next Steps

• Place outbound calls: twilio-voice-outbound-calls
• Standard IVR without AI: twilio-voice-twiml