Exotel is a cloud communications platform for voice, SMS, and WhatsApp, with data centres in Mumbai and Singapore and a focus on India, the Middle East, Africa, and APAC. For agent builders its relevant surface is AgentStream: a VoiceBot applet in the call-flow builder that opens a bidirectional WebSocket to your server, streams the caller's audio to you, and plays back whatever audio you return. That lets a speech-to-text, LLM, and text-to-speech pipeline, or a speech-to-speech model, sit on a live phone call.

Messages on the socket are JSON. Exotel sends Connected, Start (stream parameters and custom data), Media (base64 PCM chunks with sequence and timestamp), DTMF, and Stop events; your server replies with Media messages to speak, Mark to track playback completion, and Clear to drop unplayed audio. Audio is raw 16-bit little-endian mono PCM at 8 kHz by default, with 16 kHz and 24 kHz selectable by query parameter. Chunks run from 3.2 KB (100 ms) to 100 KB in multiples of 320 bytes, and a session may last up to 60 minutes.

Around the streaming layer, the Voice API connects numbers and IVR flows (POST /v1/Accounts/{sid}/Calls/connect, 200 calls per minute), applets such as Greeting, IVR Menu, Transfer, Voicemail, and Passthru compose call flows, and separate APIs cover SMS, WhatsApp, virtual numbers, and campaigns. An MCP server in beta exposes Exotel functions to agents, and hosted voice and chat agents are offered as products.

Features

AgentStream VoiceBot applet: bidirectional WebSocket audio between a live call and your bot endpoint
Event protocol: Connected, Start, Media, DTMF, Stop, Mark, and Clear messages in JSON
PCM audio: 16-bit mono little-endian at 8, 16, or 24 kHz, base64-encoded in 320-byte multiples
Session limits: up to 60 minutes per stream; the flow continues to the next applet when the socket closes
Custom parameters: up to three query parameters (256 characters total) passed to your endpoint
Voice API and applets: outbound connects, IVR menus, transfers, voicemail, and pass-through webhooks
Messaging APIs: SMS and WhatsApp alongside virtual number management
MCP server (beta): Exotel capabilities exposed to agent frameworks
Regional endpoints: api.exotel.com (Singapore) and api.in.exotel.com (Mumbai)

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