How to Build an AI Phone Agent for Inbound Calls (Step-by-Step)

This is a practical, step-by-step guide to launching an inbound AI phone agent - one that answers your business number, understands callers, answers from your own content, and books or routes as needed. You can complete the core setup in well under an hour, no code required.

Before you start

You will need three things: a phone number, a clear idea of the agent's job, and the facts it should know.

Step 1 - Get a phone number

Buy a new number or import an existing one from Twilio (or Telnyx). The platform configures the voice webhook and status callbacks on the number automatically, so incoming calls are streamed to your agent.

Step 2 - Write the system prompt

The system prompt is where you describe the agent in plain language: who it is, its tone, what it should and should not do, and how to close a call. Keep it specific. A good prompt covers:

• The agent's role and business ("You are the receptionist for Ridgeview Dental").
• Tone and length (warm, concise, one or two sentences per turn).
• What to do when unsure (offer to take a message or transfer).
• A disclosure that the caller is speaking with an AI assistant.

Step 3 - Add a knowledge base

Upload the documents the agent should answer from - hours, services, pricing, policies, FAQs - as PDF, DOCX or text. The agent retrieves from this content so answers are accurate and on-brand instead of guessed. Keep documents short and factual; break long pages into focused sections.

Step 4 - Choose a voice and speech settings

Pick a voice that fits your brand, then tune the conversational feel:

• Response eagerness - how quickly the agent jumps in after the caller stops.
• Interruption sensitivity - whether callers can barge in mid-sentence.
• Greeting - the first line the agent speaks when it answers.

Step 5 - Configure actions and fallback

Decide what the agent can do beyond talking: transfer to a human, end the call politely, detect voicemail, or capture structured details. Always define a fallback so out-of-scope calls escalate to a person or leave a message rather than dead-ending.

Step 6 - Test before you go live

Use the in-browser test call to talk to your agent with your microphone, then place a real call to the number. Listen for: does it greet correctly, answer from the knowledge base, handle an interruption, and hand off when it should? Review the transcript and recording afterward and refine the prompt.

Step 7 - Launch and monitor

Point your business line at the number and watch the first real calls. Every call is logged with its status, transcript, recording and a post-call summary, so you can spot weak spots and keep improving the prompt and knowledge base over time.

New to the concept? Start with what an AI voice agent is and how it works.