Twilio

Twilio is the dominant programmable communications platform, providing APIs for SMS, voice calls, video, WhatsApp messaging, and email that developers integrate into applications. For AI agent builders, Twilio is the most common way to give agents the ability to communicate with humans via phone calls and text messages.

Twilio's SMS API is straightforward — send a message with a POST request including a 'to' number, 'from' number, and message body. Receiving messages works via webhooks: Twilio hits your endpoint when a message arrives, and your application responds with TwiML (Twilio's XML-based markup) or a simple API call. This webhook pattern integrates naturally with agent architectures — the incoming webhook triggers the agent, and the agent's response is sent back via API.

The voice API is more complex and more powerful. Twilio handles telephony infrastructure: provisioning phone numbers, routing calls, converting speech to text, and text to speech. With the new Conversational AI features, you can build voice agents that handle phone calls with real-time speech recognition and natural language responses. The <Gather> verb captures caller input (speech or DTMF), and your application processes it through an LLM.

Twilio's Programmable Messaging supports SMS, MMS, WhatsApp, and Facebook Messenger through a unified API. This means an agent can communicate across channels without separate integrations. Message status callbacks provide delivery confirmation and error reporting.

Pricing is usage-based: ~$0.0079/SMS sent in the US, ~$0.013/minute for voice, and phone number rental at $1.15/month. Costs add up quickly for high-volume use cases — a voice agent handling 1,000 ten-minute calls costs roughly $130 in voice charges alone, plus phone number fees and any speech-to-text costs.

Twilio's moat is its reliability and global reach. It has direct carrier connections in 180+ countries, handles billions of messages, and provides the carrier-grade reliability that mission-critical communications require. The documentation is excellent, the SDKs cover every major language, and the community is enormous.

The main criticism of Twilio is pricing — it's consistently more expensive than newer competitors like Vonage or bandwidth providers. The platform has also become complex, with a sprawling product line that can be confusing to navigate. For simple SMS use cases, lighter alternatives exist, but for anything involving voice, multi-channel, or global communications, Twilio's breadth and reliability are hard to match.