1. Background & Objective

Voice AI and Contact Center (CC) platforms often operate robust SIP-based (vSIP) infrastructures for AI-led and agent-assisted conversations. However, they typically lack direct access to India’s PSTN (landline and mobile networks) due to licensing and regulatory restrictions.

Exotel, operating as a Unified License (UL) Virtual Network Operator (VNO), bridges this gap by offering compliant PSTN ingress/egress via its Virtual SIP Trunking (vSIP) solution.

Objective:
To provide a plug-and-play, regulatory-compliant PSTN connectivity layer for both Voice AI and CC platforms without disrupting their current SIP infrastructure, bot orchestration logic, or agent workflow.

  • Enable Voice AI and Contact Center platforms to access Indian PSTN via Exotel’s UL VNO.

  • Provide a plug-and-play SIP trunking layer without disrupting existing bot or agent logic.

  • Support both inbound and outbound PSTN call flows using Exotel’s vSIP infrastructure.

  • Streamline onboarding via APIs for trunk creation, DID mapping, and IP whitelisting.

This solution is ideal for:

  • Voice AI platforms using SIP-based infrastructure (e.g., FreeSWITCH, Kamailio, Asterisk, custom SBCs)

  • Contact Center platforms with on-prem or cloud SIP setups requiring PSTN termination or origination

  • Companies seeking elastic and compliant PSTN connectivity without managing telco licensing

  • AI-led and hybrid inbound/outbound voice workflows

2. Solution Overview

2.1 Architecture

  • Partner Side: Existing vSIP Endpoint / SBC / PBX (Voice AI or CC layer)

  • Exotel Side: UL VNO SIP Gateway connected to Indian PSTN via multiple telcos

Connectivity Options:

  • Public Internet (default)

  • MPLS or VPN (optional for BFSI/secure deployments; contact CSM)

Media: RTP/SRTP, supporting PCMA (G.711 A-law) and PCMU (G.711 μ-law)
Signaling: SIP over TCP or TLS

Call Flow:

  1. Outbound: Partner SBC sends SIP INVITE (only from static IP) → Exotel SIP Gateway → Indian PSTN → Customer Phone

  2. Inbound: PSTN Call → Exotel DID → SIP INVITE (from static IP or FQDN) to Partner SBC (for bot or agent)

Exotel Virtual SIP Trunking – TCP Integration Guide
Exotel Virtual SIP Trunking – TLS Integration Guide

2.2 Key Capabilities

  • Dynamic Channel Allocation for scale management

  • Multi-Telco Failover with 100% connectivity SLA

  • DID Management: Purchase & KYC compliant with Indian regulations

  • Secure Interconnects: IP whitelisting, TLS support

  • Regulatory Compliance: SDCA mapping, lawful interception readiness

Exotel Virtual SIP Trunking – To-Flow Guide

3. Onboarding & Integration

3.1 Exotel Account Setup & KYC

  1. Create Account – Sign up on Exotel and select Browser Calling during setup

  2. Complete KYC/CAF – Upload KYC Documents

3.2 SIP Trunk Enablement

  1. Enable SIP Trunking – Email hello@exotel.com to activate trunking & provision a Virtual Number/Exophone

  2. Retrieve API Credentials – Visit API Settings to get account_sid, key, and token

To configure silence ring or remove ring duration, contact your Exotel CSM or email hello@exotel.com

3.3 Trunk Configuration via API

Use:

API Steps:

  • Create Trunk (/trunks)

  • Map DIDs (/trunks/{trunk_sid}/phone-numbers)

  • Whitelist IPs (/trunks/{trunk_sid}/whitelisted-ips)

  • Add Destination URIs ( TCP, TLS)

  • Set trunk_external_alias for VN–FQDN mapping if required (for VAPI, LiveKit, etc.)

4. Inbound Call Setup (PSTN → SIP Platform)

  • Create a Flow using Connect Applet in App Bazaar

  • Use sip:<TrunkID> in the Dial Whom field

  • For custom routing, use a Dynamic URL to fetch destination URI and pass headers

  • Supports up to 3 custom SIP headers, e.g., X-param1=value1 (max 200 bytes total)

  • Headers prefixed with Exotel- or Veeno- are platform-reserved

Connect Applet – Dynamic URL Response Example:

{

  "fetch_after_attempt": false,

  "destination": { "trunk": "trunk-2134" },

  "custom_params": "param1=value1&param2=value2",

  "record": true,

  "recording_channels": "dual"

}

Reference: Connect Applet – Dynamic URL

5. Outbound Call Setup (Platform → PSTN)

5.1 Static IP Setup

  • Request IP whitelisting via Exotel Support or use above repo to get it done.

  • Once approved, initiate SIP outbound calls to Indian PSTN

5.2 FQDN-based Setup (API-triggered)

6. Voicebot to Agent Handoff (vSIP Flow)

Flow:

  • Voicebot runs on Connect Applet over vSIP

  • Bot disconnects → Flow moves to Passthru Applet → calls webhook

  • Webhook Response:

    • 200 OK: Route to Connect Applet (agent group or SIP trunk)

    • 302/404: Move to Hangup Applet

Example:
User says “Talk to human” → Bot ends → Passthru Applet → webhook (200 OK) → SwitchCase → Connect Applet (sip:<TrunkID>)

7. Benefits for Voice AI and CC Platforms

  • Plug-and-play PSTN access via Exotel’s UL VNO

  • Elastic scaling with dynamic channel allocation

  • Regulatory assurance with lawful interception readiness

  • TLS security for sensitive verticals

  • Real-time programmable call routing

8. Example Use Cases

  • AI-led outbound campaigns (sales/NPS)

  • Inbound IVR bots or reception assistants

  • Agent-bot hybrid support models

  • KYC calls and OTP verification for BFSI

  • Order confirmations and fraud checks for e-commerce

9. Next Steps

  • Share SIP endpoint details: IP/FQDN, transport type, supported codecs

  • Exotel team initiates CAF/KYC validation

  • Integration testing with live traffic

  • Commercial onboarding & production go-live

Contact: hello@exotel.com or your Exotel CSM