Passthru Applet Guide

The Passthru applet makes HTTP requests to your server during an active call flow. It enables dynamic call routing, real-time CRM lookups, data logging, and integration with any external system that exposes an HTTP API.

tip

The Passthru applet is one of Exotel's most powerful features. It transforms static call flows into dynamic, data-driven workflows. For API-level details on the Passthru applet, see the Passthru Applet API reference.

How Passthru Works

When a call reaches the Passthru applet in your flow:

1. Exotel sends an HTTP request (GET or POST) to your configured URL
2. Your server processes the request and returns a response
3. Based on the response, the call flow continues to the appropriate next applet
4. The entire round trip happens while the caller waits (typically hears hold music or silence)

Caller on hold ──► Exotel sends HTTP request to your server
                   ──► Your server processes and responds
                   ──► Exotel routes call based on response
                   ──► Caller connected to next step

Configuration

Basic Setup

1. Drag the Passthru applet onto the Flow Builder canvas
2. Click the applet to open the Properties Panel
3. Configure the following:

Setting	Required	Description
URL	Yes	The HTTP endpoint Exotel will call
HTTP Method	Yes	GET or POST
Timeout	No	Maximum time to wait for a response (default: 15 seconds)
On Timeout	No	Which applet to route to if the request times out
Hold Music	No	Audio to play while the HTTP request is in progress

Request Parameters

Exotel sends the following parameters in the HTTP request to your server:

Parameter	Description	Example
CallSid	Unique identifier for the call	abc123def456
CallFrom	Caller's phone number	+919876543210
CallTo	Called number (ExoPhone)	+914412345678
Direction	Call direction	incoming or outgoing
CurrentTime	Current timestamp	2024-01-15T10:30:00+05:30
DialWhomNumber	Number being dialed (for outbound)	+919876543210

info

For POST requests, parameters are sent as form-encoded body parameters. For GET requests, they are appended as query string parameters.

Response Format

Your server must respond with a valid HTTP response. The response body can be used to control the call flow:

HTTP/1.1 200 OK
Content-Type: application/plain

applet_name_or_url

Response-Based Routing

Your Response	Call Flow Behavior
HTTP 200 with applet URL	Route to the specified applet or external flow
HTTP 200 with empty body	Continue to the default next applet
HTTP 4xx/5xx	Route to the "On Error" path
Timeout (no response)	Route to the "On Timeout" path

Use Cases

1. CRM Lookup for VIP Routing

Look up the caller in your CRM and route VIP customers to a priority queue:

Your server logic:

Receive CallFrom → Look up in CRM database
  → If VIP customer: return "vip-connect" applet
  → If regular customer: return "standard-queue" applet
  → If unknown: return "new-customer" applet

Flow design:

Greeting ──► Passthru (CRM Lookup) ──► VIP Connect (priority agent)
                                    ──► Standard Queue (round-robin)
                                    ──► New Customer (greeting + IVR)

2. Order Status via DTMF

Collect an order number via IVR, then look up the status via Passthru:

IVR ("Enter order number") ──► Passthru (order lookup) ──► Greeting (read status via TTS)

3. Call Data Logging

Log every call to your database without affecting the flow:

Passthru (log call data) ──► Connect (proceed to agent)

Your server receives the call details, stores them, and returns HTTP 200 immediately.

4. Dynamic Business Hours Check

Check if the current time is within business hours:

Passthru (time check) ──► (business hours) Connect to agents
                       ──► (after hours) Greeting + Voicemail

5. Language Selection Based on Caller Location

Look up the caller's location and play greetings in their language:

Passthru (location lookup) ──► (Hindi) Hindi Greeting ──► Hindi IVR
                            ──► (English) English Greeting ──► English IVR
                            ──► (Tamil) Tamil Greeting ──► Tamil IVR

Server Implementation

Example: Node.js Server

// Express server handling Passthru requests
app.post('/exotel/passthru', (req, res) => {
  const callerNumber = req.body.CallFrom;
  const callSid = req.body.CallSid;

  // Look up caller in your database
  const customer = db.findByPhone(callerNumber);

  if (customer && customer.isVIP) {
    // Route to VIP flow
    res.status(200).send('https://my.exotel.com/exoml/start/vip-flow');
  } else {
    // Route to standard flow
    res.status(200).send('https://my.exotel.com/exoml/start/standard-flow');
  }
});

Server Requirements

Requirement	Details
HTTPS	Your endpoint must support HTTPS (TLS 1.2+)
Response time	Must respond within the configured timeout (default 15 seconds)
HTTP status	Return 200 for success; 4xx/5xx triggers the error path
Availability	Must be publicly accessible from Exotel's servers
Content-Type	Return text/plain or application/json

warning

If your server does not respond within the timeout period, the call is routed to the "On Timeout" path. Callers hear silence or hold music during this wait. Keep your server response time under 3 seconds for the best caller experience.

Error Handling

Configuring Fallback Paths

Always configure fallback paths for Passthru applets:

Scenario	Recommended Fallback
Server timeout	Route to a default Connect applet (general queue)
Server error (5xx)	Route to a Greeting with an apology message, then Connect
Server returns unexpected data	Route to default flow

Monitoring Passthru Performance

Track Passthru applet performance in your call logs:

• Response time: Available in CDR reports under the Passthru leg
• Success/failure rate: Filter call logs by calls that went through the error/timeout path
• Server errors: Monitor your server logs for Exotel-originated requests

Best Practices

1. Keep response times fast -- Aim for under 3 seconds; callers are waiting on the line
2. Always configure fallbacks -- Never leave the timeout or error path unconnected
3. Use hold music -- Play music while the HTTP request is in progress
4. Implement server-side logging -- Log all Passthru requests for debugging
5. Use HTTPS -- Secure your endpoint with a valid SSL certificate
6. Validate the request -- Verify that requests come from Exotel's IP ranges
7. Handle concurrent requests -- Your server must handle multiple simultaneous Passthru calls

Related Topics

• Passthru Applet API Reference -- Technical API details
• Flow Builder Guide -- Visual flow design
• Webhooks -- General webhook configuration
• Connect Applet Guide -- Route calls to agents