Flow and API Configuration Guide for Voice AI & Contact Centre Platforms
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 SIP Trunking 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:
- 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)
Exotel SIP Trunking – TCP Integration Guide Exotel 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 SIP Trunking – To-Flow Guide
3. Onboarding & Integration
3.1 Exotel Account Setup & KYC
- Create Account – Sign up on Exotel and select Browser Calling during setup
- Complete KYC/CAF – Upload KYC Documents
3.2 SIP Trunk Enablement
- Enable SIP Trunking – Email [email protected] to activate trunking & provision a Virtual Number/Exophone
- 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 [email protected]
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,OpenAI 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¶m2=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)
- Configure trunk destination via API (only for FQDN-based use cases)
- Use Click-to-Call API or Connect Customer to Flow API
- Set destination as sip:<TrunkID> in Connect Applet
6. Voicebot to Agent Handoff (SIP 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