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Flow and API Configuration Guide for Voice AI & Contact Centre Platforms via Exotel SIP Trunking

This guide provides a comprehensive, API-driven setup for Voice AI and Contact Center (CC) platforms to integrate with Exotel's SIP Trunking for compliant PSTN connectivity in India.

Background & Objective​

Voice AI and Contact Center (CC) platforms often operate robust SIP-based (SIP Trunking) 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 SIP Trunking solution.

Objectives​

  • 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 SIP Trunking infrastructure
  • Streamline onboarding via APIs for trunk creation, DID mapping, and IP whitelisting

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

Solution Overview​

Architecture​

  • Partner Side: Existing SIP Trunking 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 u-law)

Signaling: SIP over TCP or TLS

Call Flow:

  • Outbound: Partner SBC sends SIP INVITE (only from static IP) -> Exotel SIP Gateway -> Indian PSTN -> Customer Phone
  • Inbound: PSTN Call -> Exotel DID -> SIP INVITE (from static IP or FQDN) to Partner SBC (for bot or agent)
tip

For detailed SIP message format and header reference, see:

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

Onboarding & Integration​

Step 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

Step 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
note

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

Step 3: Trunk Configuration via API​

Resources:

API Steps:

  1. Create Trunk -- POST /trunks
  2. Map DIDs -- POST /trunks/{trunk_sid}/phone-numbers
  3. Whitelist IPs -- POST /trunks/{trunk_sid}/whitelisted-ips
  4. Add Destination URIs (TCP, TLS)
  5. Set trunk_external_alias for VN-FQDN mapping if required (for VAPI, LiveKit, etc.)

Inbound Call Setup (PSTN to SIP Platform)​

  1. Create a Flow using Connect Applet in App Bazaar
  2. Use sip:<TrunkID> in the Dial Whom field
  3. For custom routing, use a Dynamic URL to fetch destination URI and pass headers
  4. Supports up to 3 custom SIP headers, e.g., X-param1=value1 (max 200 bytes total)
warning

Headers prefixed with Exotel- or Veeno- are platform-reserved and cannot be used as custom headers.

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"
}

For more details, see the Connect Applet -- Dynamic URL documentation.

Outbound Call Setup (Platform to PSTN)​

Static IP Setup​

  1. Request IP whitelisting via Exotel Support or use the API repo above
  2. Once approved, initiate SIP outbound calls to Indian PSTN

FQDN-based Setup (API-triggered)​

  1. Configure trunk destination via API (only for FQDN-based use cases)
  2. Use Click-to-Call API or Connect Customer to Flow API
  3. Set destination as sip:<TrunkID> in Connect Applet

Voicebot to Agent Handoff (SIP Trunking Flow)​

Flow​

  1. Voicebot runs on Connect Applet over SIP Trunking
  2. Bot disconnects -> Flow moves to Passthru Applet -> calls webhook
  3. 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>)

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

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

Next Steps​

  1. Share SIP endpoint details: IP/FQDN, transport type, supported codecs
  2. Exotel team initiates CAF/KYC validation
  3. Integration testing with live traffic
  4. Commercial onboarding & production go-live

Contact: hello@exotel.com or your Exotel CSM