---
title: Call Directions
slug: dynamic-sip-trunking/call-directions
docTags: 
createdAt: 2026-01-24T08:58:42.723Z
---

## Call Directions

### Outbound (SIP → PSTN)

Calls initiated from your SIP system to external phone numbers.

**Call path:**

:::BlockQuote
`Your SIP System → Exotel SIP Trunk → PSTN`
:::

**Used for:**

- Agent outbound calling
- Dialers and campaigns
- Click-to-call from CRM
- Alerts and notifications

**Key requirements:**

- IP whitelisting (ACL) on the trunk
- Phone number mapped to trunk (Caller ID)

### Inbound (PSTN → SIP)

Calls initiated from the telephone network to your SIP system.

**Call path:**

:::BlockQuote
PSTN → Exotel → DID → Flow(Connect Applet) → SIP Trunk → Destination URI → Your SIP System
:::



**Used for:**

- Customer support lines
- Sales inbound numbers
- IVR and call routing to PBX

**Key requirements:**

- DID mapped to an Exotel Flow
- Flow uses **Connect Applet** with sip:\<trunk\_sid>
- Destination URI configured on the trunk (IP or FQDN)

### Bidirectional (SIP ↔ PSTN)

The same SIP Trunk is used for **both outbound and inbound calls**.

**Call paths:**

:::BlockQuote
Outbound:  Your SIP System → Exotel → PSTN
:::

:::BlockQuote
Inbound:   PSTN → Exotel → Flow(Connect Applet) → SIP Trunk → Destination URI → Your SIP System
:::

**Used for:**

- Contact centers
- PBX integrations
- Unified inbound + outbound calling
- Agent calling with inbound callbacks

**Key requirements:**

- Phone number mapped to trunk
- IP whitelisting enabled (for outbound)
- Destination URI configured (for inbound)
- Exotel Flow with Connect Applet (sip:\<trunk\_sid>)

This is the **most common SIP Trunking setup**.

## Routing Modes

Routing mode defines **how Exotel handles calls** once they reach the DID/Exophone/Virtual Number.

The mode is set while mapping a phone number to a trunk.

### mode: pstn (Default)

Standard telephony routing.

**Used for:**

- Outbound calls to PSTN
- Inbound calls routed to SIP systems
- Bidirectional SIP Trunking
- Flows using **Connect Applet** (sip:\<trunk\_sid>)

**Behavior:**

- Calls behave like standard SIP/PSTN calls
- Media flows directly between Exotel and your SIP system
- No Voice AI or Flow applet execution

### mode: flow

Flow-controlled routing.

**Used for:**

- Voicebot / AI integrations
- App Bazaar applets
- StreamKit use cases

**Behavior:**

- Calls are routed into Flow execution
- Audio may be streamed to Voice AI platforms
- Required for Voicebot and other non-Connect applets

## Mode Selection Summary

| **Use Case**            | **Flow Applet Used**        | **Mode** |
| ----------------------- | --------------------------- | -------- |
| Outbound only           | None                        | pstn     |
| Inbound only            | Connect (sip:\<trunk\_sid>) | pstn     |
| Bidirectional SIP Trunk | Connect (sip:\<trunk\_sid>) | pstn     |
| Voicebot / AI           | Voicebot / Applets          | flow     |

## Important Notes

- **Destination URI is mandatory** for all SIP Trunking setups
- **Flow is mandatory** for inbound calls
- mode controls call handling, not trunk termination

Bidirectional uses the **same trunk** for inbound and outbound