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added: calls on hold specialist transfer (#193)
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‎fern/calls/call-handling-with-vapi-and-twilio.mdx‎

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This document explains how to handle a scenario where a user is on hold while the system attempts to connect them to a specialist. If the specialist does not pick up within X seconds or if the call hits voicemail, we take an alternate action (like playing an announcement or scheduling an appointment). This solution integrates Vapi.ai for AI-driven conversations and Twilio for call bridging.
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## Problem
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Vapi.ai does not provide a built-in way to keep the user on hold, dial a specialist, and handle cases where the specialist is unavailable. We want:
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1. The user already talking to the AI (Vapi).
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2. The AI offers to connect them to a specialist.
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3. The user is placed on hold or in a conference room.
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4. We dial the specialist to join.
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5. If the specialist answers, everyone is merged.
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6. If the specialist does not answer (within X seconds or goes to voicemail), we want to either announce "Specialist not available" or schedule an appointment.
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## Solution
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1. An inbound call arrives from Vapi or from the user directly.
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2. We store its details (e.g., Twilio CallSid).
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3. We send TwiML (or instructions) to put the user in a Twilio conference (on hold).
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4. We place a second call to the specialist, also directed to join the same conference.
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5. If the specialist picks up, Twilio merges the calls.
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6. If not, we handle the no-answer event by playing a message or returning control to the AI for scheduling.
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## Steps to Solve the Problem
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1. **Receive Inbound Call**
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- Twilio posts data to your `/inbound_call`.
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- You store the call reference.
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- You might also invoke Vapi for initial AI instructions.
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2. **Prompt User via Vapi**
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- The user decides whether they want the specialist.
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- If yes, you call an endpoint (e.g., `/connect`).
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3. **Create/Join Conference**
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- In `/connect`, you update the inbound call to go into a conference route.
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- The user is effectively on hold.
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4. **Dial Specialist**
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- You create a second call leg to the specialist’s phone.
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- A `statusCallback` can detect no-answer or voicemail.
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5. **Detect Unanswered**
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- If Twilio sees a no-answer or failure, your callback logic plays an announcement or signals the AI to schedule an appointment.
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6. **Merge or Exit**
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- If the specialist answers, they join the user.
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- If not, the user is taken off hold and the call ends or goes back to AI.
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7. **Use Ephemeral Call (Optional)**
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- If you need an in-conference announcement, create a short-lived Twilio call that `<Say>` the message to everyone, then ends the conference.
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## Code Example
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Below is a minimal Express.js server aligned for On-Hold Specialist Transfer with Vapi and Twilio.
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1. **Express Setup and Environment**
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```js
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const express = require("express");
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const bodyParser = require("body-parser");
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const axios = require("axios");
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const twilio = require("twilio");
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const app = express();
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app.use(bodyParser.urlencoded({ extended: true }));
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app.use(bodyParser.json());
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// Load important env vars
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const {
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TWILIO_ACCOUNT_SID,
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TWILIO_AUTH_TOKEN,
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FROM_NUMBER,
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TO_NUMBER,
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VAPI_BASE_URL,
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PHONE_NUMBER_ID,
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ASSISTANT_ID,
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PRIVATE_API_KEY,
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} = process.env;
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// Create a Twilio client
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const client = twilio(TWILIO_ACCOUNT_SID, TWILIO_AUTH_TOKEN);
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// We'll store the inbound call SID here for simplicity
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let globalCallSid = "";
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```
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2. **`/inbound_call` - Handling the Inbound Call**
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```js
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app.post("/inbound_call", async (req, res) => {
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try {
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globalCallSid = req.body.CallSid;
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const caller = req.body.Caller;
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// Example: We call Vapi.ai to get initial TwiML
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const response = await axios.post(
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`${VAPI_BASE_URL || "https://api.vapi.ai"}/call`,
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{
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phoneNumberId: PHONE_NUMBER_ID,
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phoneCallProviderBypassEnabled: true,
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customer: { number: caller },
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assistantId: ASSISTANT_ID,
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},
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{
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headers: {
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Authorization: `Bearer ${PRIVATE_API_KEY}`,
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"Content-Type": "application/json",
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},
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}
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);
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const returnedTwiml = response.data.phoneCallProviderDetails.twiml;
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return res.type("text/xml").send(returnedTwiml);
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} catch (err) {
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return res.status(500).send("Internal Server Error");
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}
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});
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```
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3. **`/connect` - Putting User on Hold and Dialing Specialist**
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```js
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app.post("/connect", async (req, res) => {
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try {
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const protocol =
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req.headers["x-forwarded-proto"] === "https" ? "https" : "http";
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const baseUrl = `${protocol}://${req.get("host")}`;
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const conferenceUrl = `${baseUrl}/conference`;
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// 1) Update inbound call to fetch TwiML from /conference
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await client.calls(globalCallSid).update({
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url: conferenceUrl,
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method: "POST",
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});
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// 2) Dial the specialist
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const statusCallbackUrl = `${baseUrl}/participant-status`;
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await client.calls.create({
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to: TO_NUMBER,
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from: FROM_NUMBER,
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url: conferenceUrl,
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method: "POST",
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statusCallback: statusCallbackUrl,
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statusCallbackMethod: "POST",
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});
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return res.json({ status: "Specialist call initiated" });
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} catch (err) {
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return res.status(500).json({ error: "Failed to connect specialist" });
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}
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});
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```
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4. **`/conference` - Placing Callers Into a Conference**
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```js
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app.post("/conference", (req, res) => {
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const VoiceResponse = twilio.twiml.VoiceResponse;
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const twiml = new VoiceResponse();
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// Put the caller(s) into a conference
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const dial = twiml.dial();
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dial.conference(
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{
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startConferenceOnEnter: true,
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endConferenceOnExit: true,
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},
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"my_conference_room"
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);
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return res.type("text/xml").send(twiml.toString());
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});
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```
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5. **`/participant-status` - Handling No-Answer or Busy**
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```js
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app.post("/participant-status", async (req, res) => {
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const callStatus = req.body.CallStatus;
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if (["no-answer", "busy", "failed"].includes(callStatus)) {
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console.log("Specialist did not pick up:", callStatus);
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// Additional logic: schedule an appointment, ephemeral call, etc.
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}
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return res.sendStatus(200);
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});
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```
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6. **`/announce` (Optional) - Ephemeral Announcement**
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```js
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app.post("/announce", (req, res) => {
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const VoiceResponse = twilio.twiml.VoiceResponse;
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const twiml = new VoiceResponse();
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twiml.say("Specialist is not available. Ending call now.");
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// Join the conference, then end it.
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twiml.dial().conference(
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{
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startConferenceOnEnter: true,
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endConferenceOnExit: true,
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},
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"my_conference_room"
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);
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return res.type("text/xml").send(twiml.toString());
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});
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```
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7. **Starting the Server**
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```js
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app.listen(3000, () => {
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console.log("Server running on port 3000");
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});
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```
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## How to Test
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1. **Environment Variables**
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Set `TWILIO_ACCOUNT_SID`, `TWILIO_AUTH_TOKEN`, `FROM_NUMBER`, `TO_NUMBER`, `VAPI_BASE_URL`, `PHONE_NUMBER_ID`, `ASSISTANT_ID`, and `PRIVATE_API_KEY`.
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2. **Expose Your Server**
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- Use a tool like `ngrok` to create a public URL to port 3000.
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- Configure your Twilio phone number to call `/inbound_call` when a call comes in.
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3. **Place a Real Call**
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- Dial your Twilio number from a phone.
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- Twilio hits `/inbound_call`, and run Vapi logic.
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- Trigger `/connect` to conference the user and dial the specialist.
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- If the specialist answers, they join the same conference.
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- If they never answer, Twilio eventually calls `/participant-status`.
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4. **Use cURL for Testing**
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- **Simulate Inbound**:
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```bash
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curl -X POST https://<public-url>/inbound_call \
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-F "CallSid=CA12345" \
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-F "Caller=+15551112222"
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```
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- **Connect**:
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```bash
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curl -X POST https://<public-url>/connect \
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-H "Content-Type: application/json" \
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-d "{}"
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```
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## Note on Replacing "Connect" with Vapi Tools
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Vapi offers built-in functions or custom tool calls for placing a second call or transferring, you can replace the manual `/connect` call with that Vapi functionality. The flow remains the same: user is put in a Twilio conference, the specialist is dialed, and any no-answer events are handled.
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## Notes & Limitations
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1. **Voicemail**
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If a phone’s voicemail picks up, Twilio sees it as answered. Consider advanced detection or a fallback.
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2. **Concurrent Calls**
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Multiple calls at once require storing separate `CallSid`s or similar references.
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3. **Conference Behavior**
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`startConferenceOnEnter: true` merges participants immediately; `endConferenceOnExit: true` ends the conference when that participant leaves.
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4. **X Seconds**
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Decide how you detect no-answer. Typically, Twilio sets a final `callStatus` if the remote side never picks up.
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With these steps and code, you can integrate Vapi Assistant while using Twilio’s conferencing features to hold, dial out to a specialist, and handle an unanswered or unavailable specialist scenario.

‎fern/docs.yml‎

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path: calls/call-ended-reason.mdx
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- page: Live Call Control
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path: calls/call-features.mdx
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- page: On-Hold Specialist Transfer
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path: calls/call-handling-with-vapi-and-twilio.mdx
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- page: Voice Mail Detection
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path: calls/voice-mail-detection.mdx
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- section: SIP

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