Answer Webhook

The Answer Webhook is called when a call is answered (callee picks up the phone). Your server must respond with a Stream JSON configuration that tells Superfone where to stream the call's audio.

Critical

This webhook MUST return a valid Stream JSON response within 10 seconds, or the call will fail. If the primary answer_url fails, Superfone tries the fallback_answer_url (if configured).

When It's Called

The answer webhook is triggered when:

• Inbound calls: A caller dials a number linked to your SFVoPI app, and the call is answered
• Outbound calls: You initiate a call via the API, and the callee answers

Webhook Payload

Superfone sends a POST or GET request (based on your app configuration) with the following payload:

Payload Structure (FLAT)

Important

The payload is FLAT — all fields are at the top level, including request_uuid for your convenience.

Field	Type	Description
call_uuid	string	Unique identifier for this call
request_uuid	string	Request UUID for outbound calls (format: sfv_ob_req_xxxxxxxxxxxx). Not present for inbound calls.
from	string	Caller's phone number (E.164 format, e.g., +918000000003)
to	string	Callee's phone number (E.164 format, e.g., +918000000001)
direction	string	Call direction: INBOUND or OUTBOUND
call_status	string	Current call status: IN_PROGRESS
answered_at	string	ISO 8601 timestamp when call was answered (e.g., 2026-02-02T10:00:00.000Z)

Example Payloads

Outbound Call

{
  "call_uuid": "call-uuid-1738491600-abc123",
  "request_uuid": "sfv_ob_req_a8k3m2x9p1z0",
  "from": "+918000000003",
  "to": "+918000000001",
  "direction": "OUTBOUND",
  "call_status": "IN_PROGRESS",
  "answered_at": "2026-02-02T10:00:00.000Z"
}

Inbound Call

{
  "call_uuid": "call-uuid-1738491600-def456",
  "from": "+918000000001",
  "to": "+918000000003",
  "direction": "INBOUND",
  "call_status": "IN_PROGRESS",
  "answered_at": "2026-02-02T10:00:00.000Z"
}

info

Inbound calls do not include request_uuid because they are not initiated via the API.

Required Response: Stream JSON

Your server must respond with HTTP 200 and a JSON body containing the stream object:

Response Schema

{
  "stream": {
    "url": "wss://your-server.com/ws",
    "codec": "PCMA",
    "sample_rate": 8000,
    "direction": "BOTH",
    "bidirectional": true,
    "stream_timeout": 86400,
    "keep_call_alive": true,
    "extra_headers": {
      "X-Session-Id": "abc123",
      "X-Custom-Header": "value"
    }
  }
}

Stream Object Fields

Field	Type	Required	Default	Description	Validation
url	string	Yes	—	WebSocket URL where Superfone will stream audio (must start with wss:// or ws://)	Valid WebSocket URL
codec	string	Yes	PCMA	Audio codec: PCMA (G.711 A-law, recommended), PCMU (G.711 μ-law), or L16 (16-bit linear PCM)	Enum: PCMA, PCMU, L16
sample_rate	number	Yes	8000	Audio sample rate in Hz. PCMA/PCMU: 8000 only. L16: 8000, 16000, 24000, 32000, 44100, 48000.	See description
direction	string	Yes	BOTH	Audio stream direction: INBOUND (from callee), OUTBOUND (to callee), or BOTH (bidirectional)	Enum: INBOUND, OUTBOUND, BOTH
bidirectional	boolean	No	true	Whether audio flows in both directions (deprecated — use direction instead)	Boolean
stream_timeout	number	No	86400	Maximum stream duration in seconds (1-86400, i.e., 1 second to 24 hours)	Min: 1, Max: 86400
keep_call_alive	boolean	No	true	Whether to keep call active after stream ends	Boolean
extra_headers	object	No	{}	Custom HTTP headers to send in WebSocket handshake request	Key-value pairs (string keys, string values)

Recommended: codec: "PCMA", sample_rate: 8000

The SFVoPI telephony leg runs A-law @ 8 kHz natively. Picking PCMA + 8000 makes SFVoPI a pure passthrough — no transcoding, lowest latency. Use L16 only if your pipeline specifically needs raw linear PCM over the wire. Avoid PCMU (forces a pointless A-law ↔ μ-law roundtrip). Do not request sample rates > 8000 — the trunk is 8 kHz, anything higher is upsampled on SFVoPI's side for no real quality gain.

tip

Use extra_headers to pass session identifiers, authentication tokens, or other metadata to your WebSocket server.

Webhook Handler Examples

JavaScript

const express = require('express');
const app = express();

app.use(express.json());

app.post('/webhook/answer', (req, res) => {
  const payload = req.body;
  
  console.log('[Answer Webhook] Call answered:', {
    call_uuid: payload.call_uuid,
    request_uuid: payload.request_uuid,
    from: payload.from,
    to: payload.to,
    direction: payload.direction,
    answered_at: payload.answered_at
  });
  
  // Return Stream JSON
  const streamConfig = {
    stream: {
      url: 'wss://your-server.com/ws',
      codec: 'PCMA',
      sample_rate: 8000,
      direction: 'BOTH',
      stream_timeout: 86400,
      extra_headers: {
        'X-Call-UUID': payload.call_uuid,
        'X-Request-UUID': payload.request_uuid || 'N/A'
      }
    }
  };
  
  res.json(streamConfig);
});

app.listen(3000, () => {
  console.log('Webhook server listening on port 3000');
});

TypeScript

import express, { Request, Response } from 'express';

interface AnswerWebhookPayload {
  call_uuid: string;
  request_uuid?: string;
  from: string;
  to: string;
  direction: 'INBOUND' | 'OUTBOUND';
  call_status: 'IN_PROGRESS';
  answered_at: string;
}

interface StreamResponse {
  stream: {
    url: string;
    codec: 'PCMA' | 'PCMU' | 'L16';
    sample_rate: 8000 | 16000 | 24000 | 32000 | 44100 | 48000;
    direction: 'INBOUND' | 'OUTBOUND' | 'BOTH';
    bidirectional?: boolean;
    stream_timeout?: number;
    keep_call_alive?: boolean;
    extra_headers?: Record<string, string>;
  };
}

const app = express();
app.use(express.json());

app.post('/webhook/answer', (req: Request, res: Response) => {
  const payload = req.body as AnswerWebhookPayload;
  
  console.log('[Answer Webhook] Call answered:', {
    call_uuid: payload.call_uuid,
    request_uuid: payload.request_uuid,
    from: payload.from,
    to: payload.to,
    direction: payload.direction,
    answered_at: payload.answered_at
  });
  
  // Return Stream JSON
  const streamConfig: StreamResponse = {
    stream: {
      url: 'wss://your-server.com/ws',
      codec: 'PCMA',
      sample_rate: 8000,
      direction: 'BOTH',
      stream_timeout: 86400,
      extra_headers: {
        'X-Call-UUID': payload.call_uuid,
        'X-Request-UUID': payload.request_uuid || 'N/A'
      }
    }
  };
  
  res.json(streamConfig);
});

app.listen(3000, () => {
  console.log('Webhook server listening on port 3000');
});

Python

from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/webhook/answer', methods=['POST'])
def answer_webhook():
    payload = request.json
    
    print('[Answer Webhook] Call answered:', {
        'call_uuid': payload['call_uuid'],
        'request_uuid': payload.get('request_uuid', 'N/A'),
        'from': payload['from'],
        'to': payload['to'],
        'direction': payload['direction'],
        'answered_at': payload['answered_at']
    })
    
    # Return Stream JSON
    stream_config = {
        'stream': {
            'url': 'wss://your-server.com/ws',
            'codec': 'PCMA',
            'sample_rate': 8000,
            'direction': 'BOTH',
            'stream_timeout': 86400,
            'extra_headers': {
                'X-Call-UUID': payload['call_uuid'],
                'X-Request-UUID': payload.get('request_uuid', 'N/A')
            }
        }
    }
    
    return jsonify(stream_config)

if __name__ == '__main__':
    app.run(port=3000)

Timeout and Fallback

Primary Answer URL

Superfone waits 10 seconds for your answer_url to respond. If it fails (timeout, connection error, non-200 status, invalid JSON), Superfone tries the fallback_answer_url (if configured).

Fallback Answer URL

If the primary answer_url fails, Superfone tries the fallback_answer_url with a 5-second timeout. If the fallback also fails, the call is terminated with FAILED status.

Failure Scenarios

Scenario	Behavior
Primary succeeds	Call proceeds with Stream JSON from primary
Primary fails, fallback succeeds	Call proceeds with Stream JSON from fallback
Both fail	Call terminated, hangup webhook sent with call_status: "FAILED"

tip

Configure a fallback_answer_url pointing to a backup server or a static WebSocket endpoint to ensure high availability.

Error Responses

If your webhook returns an error, Superfone logs it and tries the fallback (if configured):

Status Code	Meaning	Superfone Action
200	Success — Stream JSON accepted	Proceed with audio streaming
400	Bad Request — Invalid Stream JSON	Try fallback, then fail call
500	Internal Server Error	Try fallback, then fail call
Timeout	No response within 10 seconds	Try fallback, then fail call

Invalid Stream JSON Examples

// Missing required field 'url'
{
  "stream": {
    "codec": "PCMU",
    "sample_rate": 8000,
    "direction": "BOTH"
  }
}

// Invalid codec value
{
  "stream": {
    "url": "wss://your-server.com/ws",
    "codec": "OPUS",
    "sample_rate": 8000,
    "direction": "BOTH"
  }
}

// Invalid sample_rate value
{
  "stream": {
    "url": "wss://your-server.com/ws",
    "codec": "PCMU",
    "sample_rate": 48000,
    "direction": "BOTH"
  }
}

Use Cases

1. Dynamic WebSocket Routing

Route calls to different WebSocket servers based on call metadata:

app.post('/webhook/answer', (req, res) => {
  const { from, to, direction } = req.body;
  
  // Route to different servers based on direction
  const wsUrl = direction === 'INBOUND'
    ? 'wss://inbound-server.com/ws'
    : 'wss://outbound-server.com/ws';
  
  res.json({
    stream: {
      url: wsUrl,
      codec: 'PCMA',
      sample_rate: 8000,
      direction: 'BOTH'
    }
  });
});

2. Session Tracking with Extra Headers

Pass session identifiers to your WebSocket server:

app.post('/webhook/answer', (req, res) => {
  const { call_uuid, request_uuid } = req.body;
  
  // Generate session ID
  const sessionId = `session_${Date.now()}_${call_uuid}`;
  
  res.json({
    stream: {
      url: 'wss://your-server.com/ws',
      codec: 'PCMA',
      sample_rate: 8000,
      direction: 'BOTH',
      extra_headers: {
        'X-Session-Id': sessionId,
        'X-Call-UUID': call_uuid,
        'X-Request-UUID': request_uuid || 'N/A'
      }
    }
  });
});

3. Call Time Limits

Set a custom stream timeout based on call type:

app.post('/webhook/answer', (req, res) => {
  const { direction } = req.body;
  
  // Shorter timeout for outbound calls
  const timeout = direction === 'OUTBOUND' ? 300 : 3600;
  
  res.json({
    stream: {
      url: 'wss://your-server.com/ws',
      codec: 'PCMA',
      sample_rate: 8000,
      direction: 'BOTH',
      stream_timeout: timeout
    }
  });
});

Notes

1. Response Time: Respond as quickly as possible (ideally <1 second) to minimize call setup delay
2. WebSocket Availability: Ensure your WebSocket server is running and reachable before returning the Stream JSON
3. HTTPS Required: Use wss:// (secure WebSocket) in production; ws:// is only for local development
4. Idempotency: Use call_uuid to track processed calls and avoid duplicate processing
5. Logging: Log all webhook requests for debugging and audit purposes
6. Error Handling: Return HTTP 200 even if you encounter internal errors — log the error and return a fallback Stream JSON

Related Endpoints

• Hangup Webhook — Handle call ended events
• Audio Streaming Overview — Learn how to stream audio via WebSocket
• WebSocket Protocol — Understand WebSocket message format
• Create App API — Configure answer webhook URL
• Initiate Call API — Override answer webhook per call

Next Steps

• Implement WebSocket Server — Handle audio streaming after answer webhook
• Hangup Webhook — Process call end events and CDR data
• Handle WebSocket disconnections and errors