Resources›Retell AI›Retell AI: Build AI Phone Agents That Handle Calls End-to-End
Guide6 min read·Updated April 10, 2026

Retell AI: Build AI Phone Agents That Handle Calls End-to-End

Retell AI powers AI voice agents for inbound and outbound phone calls — 600ms response time, 31+ languages, 40M+ calls per month. Here's how to build your first agent.

Retell AI: Build AI Phone Agents That Handle Calls End-to-End

Retell AI is a voice agent platform for phone call automation. It handles the full call experience — conversation logic, voice synthesis, telephony, and integrations — so you can deploy an AI that answers (or makes) phone calls like a human agent.

Scale that validates it: $40M ARR, 40M+ real phone calls per month as of January 2026.

What Retell AI Handles

  • Inbound calls — AI answers your business line, qualifies leads, books appointments, answers FAQs, routes to humans when needed
  • Outbound campaigns — AI calls a list of prospects, delivers a script, handles objections, books meetings
  • Omnichannel — Voice, chat, email, and SMS in a unified platform
  • 31+ languages — Serve multilingual markets without multilingual staff

Why Phone AI Now

The average business can't staff an inbound line 24/7. An after-hours call that goes to voicemail is typically a lost lead — most people don't leave messages, and response time is critical for conversion.

Retell's agents respond in 600ms — faster than most human agents — and are available every hour of every day. For businesses where calls are a primary acquisition channel (real estate, home services, healthcare, insurance, agencies), this is a direct revenue impact.

Three Agent Types

1. Single Prompt Agent The simplest setup. Write a system prompt describing your agent's persona, knowledge, and behavior. Good for customer service FAQs and simple lead capture.

2. Conversation Flow Agent A structured flowchart-style conversation — predefined paths based on caller responses. Best for appointment booking and structured qualification where you need predictable outcomes.

3. Custom LLM Agent Connect your own LLM (GPT-4o, Claude, or any compatible model via API). Full control over the AI logic — for teams with specific requirements or existing AI infrastructure.

Getting Started

  1. Sign up at retellai.com
  2. Click Create Agent and choose your agent type
  3. Write your agent prompt or build your conversation flow
  4. Connect a phone number (purchase within Retell or bring your own via Twilio)
  5. Test by calling the number — the agent is live immediately

Example agent prompt (inbound lead capture):

You are Alex, a friendly assistant at Lakeside Landscaping. When someone calls:
1. Greet them and ask how you can help
2. If they're interested in a quote: collect their name, address, and what service they need
3. Offer to book a free estimate — check availability and confirm a time
4. If it's outside business hours, let them know someone will follow up by 9am

Always be warm, professional, and concise. Never promise prices.

Retell Assure: Automated QA

Launched January 1, 2026. Retell Assure automatically monitors call quality, flags issues, and suggests improvements — without someone having to listen to every call. For businesses running high call volume, manual quality review doesn't scale. Assure makes QA automatic.

GPT-4.1 Integration

Retell's agents now run GPT-4.1 by default — faster responses, better conversation management, smarter handling of off-script situations. You can also connect Claude or your own fine-tuned model.

Retell AI + ElevenLabs

Retell handles the conversation logic and telephony. ElevenLabs handles the voice quality. If you want a custom cloned voice for your agent, ElevenLabs is the natural pairing:

  1. Clone your target voice in ElevenLabs
  2. Get the ElevenLabs voice ID
  3. Set it as your Retell agent's voice in the Voice settings

This gives you a conversation agent that sounds like a real, specific person — useful for brand consistency.

CRM and API Integration

Retell connects to existing CRM and telephony systems via REST API. Common integrations:

  • CRM — Create/update contact records after calls (HubSpot, Salesforce, GoHighLevel)
  • Scheduling — Book appointments directly in Calendly, Acuity, or Google Calendar
  • Notifications — Send Slack messages or emails when calls complete with summaries

Learning Resources

  • Official Docs — docs.retellai.com/general/introduction
  • Official Site — retellai.com
  • How to Build AI Agents for Beginners (Retell Blog) — Agent types and when to use each
  • Training & Customizing Voice Agents (Retell Blog) — Prompting and behavior customization guide
  • How to Integrate with Your API (Retell Blog) — For connecting to CRM and external systems
  • Retell AI Discord — Primary community hub for builders
Back to Retell AI