What Is Retell AI? AI Voice Agents Explained
Retell AI (often just called Retell) is a platform for building AI voice agents — software that answers the phone, holds a natural conversation, and takes real actions like booking appointments. Here's what it is, how it works, what it costs and what you can build with it.
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Quick answer
Retell AI is a platform for building AI voice agents — software that answers and makes phone calls, holds a natural conversation and takes actions such as booking an appointment or updating a CRM. Retell runs the speech recognition, the language model, the voice and the phone line; you configure the agent's instructions, functions and integrations. It's pay-as-you-go: Retell lists $0.07–$0.31 per minute depending on the model, voice and telephony you choose, and new accounts get $10 of free credit.
How Retell AI works
It combines speech, an AI brain and your tools into a voice agent that talks and acts.
- Natural, low-latency voice conversations
- Understands intent and responds in real time
- Takes actions: booking, routing, data capture
- Connects to your calendar, CRM and APIs
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What you configure
Retell provides the infrastructure; the quality of the agent comes from how it's set up.
- The agent's instructions — a single prompt, or a conversation flow with defined steps
- The language model (GPT, Claude, Gemini and others) and the voice
- Functions that let it act: check availability, book, transfer the call, send data to a webhook
- A knowledge base so it answers from your own information
- Phone numbers — bought through Retell or connected from your own provider
Is Retell AI free?
Not beyond a starter credit. Retell is pay-as-you-go: new accounts get $10 of free credit and 20 concurrent calls, and after that every minute is billed. Test calls cost the same as real ones, so the free credit is best spent testing a short, focused agent.
- Typical all-in cost with GPT-4.1 and a standard voice on a Retell number: about $0.13 a minute
- Phone numbers bought through Retell: $2 a month each
- Full breakdown in my Retell AI pricing guide
What you can build
Real, useful voice agents — not gimmicks.
- AI receptionists and booking agents
- Lead qualification by phone
- After-hours and overflow answering
- Outbound call campaigns
A real example
I built an after-hours receptionist on Retell for a walk-in dental clinic in Tampa, Florida. It answers in English or Spanish, triages emergencies first, checks live availability in the clinic's booking system, and captures details for a morning callback when it can't book — with guardrails that stop it confirming anything it hasn't verified.