How to Set Up Retell AI: Step-by-Step Tutorial for Your First Voice Agent

Before you start

You'll need a Retell AI account (free to start), a Twilio account if you want a real phone number, and a clear idea of what the agent should do. Spend 10 minutes writing out the conversation in plain English before you touch the dashboard.

Step 1 — Create your Retell account

Go to retellai.com and sign up. You get free test minutes immediately. Add a payment method when you're ready to attach a phone number — Retell won't let you buy or import numbers without one.

Step 2 — Build your first agent

Click "Create Agent". Pick the "Single Prompt" template for your first one — it's the easiest to understand. The prompt is the entire personality and behavior. Keep it short. State the role, the goal, the tone, and the rules. That's it.

Step 3 — Pick a voice

Retell ships with strong stock voices. "Cimo" and "Anna" are crowd favorites. Don't reach for ElevenLabs on day one — you'll add cost and latency for a difference your customers won't notice.

Step 4 — Connect a phone number

Buy a number directly inside Retell ($2/mo) or import a Twilio number you already own. To import: in Twilio, go to your number's config, set the voice webhook to the Retell-provided URL, then in Retell paste the Twilio SID and auth token.

Step 5 — Add function calls

Functions are how the agent does things — book a meeting, look up an order, send a text. In Retell, add a "Custom Function", give it a webhook URL pointing at your backend (or a no-code tool like Make.com), and describe when to call it. The description matters more than the schema — the LLM uses it to decide.

Step 6 — Test and go live

Use the in-dashboard test call before anything else. Then call your real number from your phone. Then have a friend call. Then have a friend who's bad at following directions call — they'll break it, and that's the point. Fix the prompt, ship it.

Common mistakes

• Prompts longer than one page. Cut them in half.
• No "end call" condition — agents will ramble forever.
• Skipping the voicemail detection setting on outbound agents.
• Forgetting to set max call duration as a budget guardrail.