***

title: Forward calls to the SIP trunk
subtitle: null
slug: forward-calls-to-sip-trunk
description: null
-----------------

This guide walks you through the process of initializing the call via the SIP trunk. The setup consists of three primary steps:

1. **Create the call**
2. **Forward call to SIP Trunk**

***

## Final Step: Initializing the call

### Creating the call

**Endpoint:**\
`POST /v2/prepare_inbound`

**Headers:**

```Text JSON
{
  "Content-Type": "application/json",
  "Authorization": "Bearer {TOKEN}"
}
```

**Request Body**

```Text JSON
{
  "from_number": "",
  "to_number": "",
  "model_id": "",
  "workspace_id": "",
  "custom_variables": {
    "name": "value"
  },
}
```

**Response Format**

```Text JSON
{
  "call_status": "registered",
  "synthflow_call_id": "a29407fb-1bf2-4be6-9d1d-f501c3dd9a2b",
  "direction": "inbound",
  "model_id": "1744274157255x646200691892837600",
  "workspace_id": null,
  "custom_variables": {
    "name": "erfan",
    "job": "backend"
  },
  "duration": null,
  "end_call_reason": null,
  "judge_results": null,
  "is_voicemail": null,
  "lead_phone_number": "+15075785607",
  "voicemail_not_detected_during_call_but_detected_in_post_call": null,
  "voicemail_during_call_but_not_voicemail_in_post_call": null,
  "timeline": [
    
  ],
  "twilio_call_sid": null,
  "egress_recording_url": null,
  "agents_used": [
    
  ],
  "original_payload": null,
  "executed_actions": [
    
  ],
  "prompt_variables": {
    
  },
  "voicemail_message_left": null,
  "recording_sid": null,
  "secondary_call_id": null,
  "recording_url": null,
  "payload_received": null,
  "language": null,
  "type_of_call": null,
  "recording_duration": null,
  "transcript": null,
  "error_message_payload_validation": null,
  "error_message": null,
  "node_and_stage_time_track": {
    
  },
  "start_time": 1744309477691,
  "campaign_name": null,
  "lead_name": null,
  "labels": [
    
  ],
  "agent_phone_number": "",
  "deployment_type": null,
  "timezone": null
}
```

**synthflow\_call\_id** This is the call ID that should be sent to the SIP trunk via the X-EI header.

**call\_status** Indicates the status of the call. A value of registered means that SynthFlow is operational, while failed indicates a service outage.

### Forward call to SIP Trunk

After the call is created on SynthFlow, redirect Twilio calls to it using TwiML commands. Make sure to include the received `synthflow_call_id` in the SIP headers using the `X-EI` key, formatted as follows:

X-EI=S.`synthflow_call_id`;

Regex pattern: `(S|E)\.([^\;]+);`

Here’s a code snippet for redirecting a call:

```javascript
const express = require('express');
const VoiceResponse = require('twilio').twiml.VoiceResponse;

const app = express();
const PORT = process.env.PORT || 3000;
const TERMINATION = "sip.synthflow.ai:32681";

app.get('/redirect_call', (req, res) => {
  const phone_number = "" // Custom phone number
  const call_id = "" // synthflow_call_id
  
  const voice = new VoiceResponse();
  voice.dial().sip(`sip:${phone_number}@${TERMINATION}?X-EI=S.${call_id};`);
  
  res.type('text/xml');
  res.send(voice.toString());
});

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});
```

```Text python
from fastapi import FastAPI, Response, Request

app = FastAPI()

TERMINATION = "sip.synthflow.ai:32681"

@app.get("/redirect_call")
async def redirect_call(CallSid: str, From: str):
    phone_number = "" # Custom Phone number
    call_id = await create_call()  # Creating call and returning synthflow_call_id

    voice = VoiceResponse()
    voice.dial().sip(f"sip:{phone_number}@{TERMINATION}?X-EI=S.{call_id};")
    return Response(str(voice), media_type="text/xml", status_code=200)
```

***

> Note: The transfer call will work the same as before.