Building Real Voice AI Agents with Vapi

Voice AI is evolving fast — and Vapi is one of the most developer-friendly platforms making it real. If you've ever wanted to build an AI that can talk, transfer, and think, here's the complete breakdown you need.

Building Real Voice AI Agents with Vapi

The future of AI assistants isn't chat-based — it's voice-based. And platforms like Vapi are making that future easier to build than ever.

1. What is an Assistant in Vapi?

Think of it as your AI voice agent. An Assistant is an AI that listens, understands, and responds to users during a call naturally — handling users just like a real agent would.

Key Features:

  • Listens to caller input in real-time
  • Understands natural language
  • Responds conversationally
  • Executes tasks during the call

2. System Prompt — The Assistant's Personality

The System Prompt is the main instruction given to the AI model. It tells the assistant how to talk, what tone to use, and what it should do.

Why it's used:

  • Controls the assistant's behavior
  • Makes responses consistent and professional
  • Ensures it follows specific rules (like asking for name, phone number, etc.)
  • Sets the tone, style, and logic

The system prompt defines your assistant's personality — whether it's friendly, professional, or technical. It's the foundation of how your AI interacts with callers.

3. Tools — Where AI Starts Doing Real Work

Tools are small actions the assistant can perform automatically. They help the assistant do real tasks — not just talk.

Common Tools:

  1. Query Tool - Searches information from knowledge bases
  2. Transfer Call Tool - Connects caller to a real human

3.1 How to Link Multiple Tools to the Assistant

  1. Go to Assistant module
  2. Click the Assistant and select the "Tools" tab
  3. At the bottom, select the Tools you want to use for this assistant
  4. Click Publish to save

You can attach multiple tools per assistant to enhance its capabilities and handle different scenarios.

4. Query Tool — The Knowledge Engine

Purpose: Lets assistant search information from uploaded Knowledge Base files.

How it works:

  1. Caller asks a question
  2. Assistant searches knowledge base files
  3. Returns relevant information
  4. Assistant responds to caller with accurate data

4.1 What are Knowledge Base Files?

Knowledge Base Files are reference documents (PDF, TXT) that help the assistant give correct answers about your business or process.

Example: A law firm can upload "Accepted Case Types.pdf". The assistant reads it and uses it to answer related questions correctly.

Purpose:

  • Provide background information
  • Help the assistant give accurate, company-specific answers
  • Make your AI genuinely knowledge-aware, not just smart-sounding

4.2 How To Upload Knowledge Base Files in Vapi

  1. Go to the Files Module
  2. Upload the new Knowledge Base .txt file
  3. If you need to edit existing files:
    • Download the existing file
    • Delete the existing Knowledge Base file
    • Reupload the corrected Knowledge Base file again

4.3 Where to Link Knowledge Base Files

  1. Go to the Query Tool
  2. Under Knowledge Bases section, add multiple knowledge base files
  3. Save and publish

5. Transfer Tool — When AI Meets Human

In simple words, it's designed to make a smooth hand-off from AI to a real person. The Transfer Call Tool lets your AI assistant connect the caller to a real human agent.

It is used when:

  • The caller needs help from a live person
  • The AI has finished collecting the needed details
  • The query is too complex for automated handling

5.1 How to Add Phone Numbers for Transfer Call Tool

  1. Go to Tools in the Vapi dashboard
  2. Click Create Tool and select Transfer Call Tool
  3. Enter the name and description of the tool
  4. Scroll to the Destination section
  5. Click Add Phone Number to add your real phone numbers
  6. Make sure each number is in the correct E.164 format (example: +15012488744)
  7. Enable Enforce E164 format for each phone number
  8. Add multiple phone numbers if needed
  9. Click Save Tool to finish

6. Phone Number Integration

Purpose: The phone number in Vapi is used to make and receive real calls through the assistant. It connects the AI assistant to actual phone networks, allowing inbound calls from users and outbound or transfer calls to real people.

6.1 Why We Need to Use Twilio Phone Numbers

  • Twilio phone numbers are reliable and ready for production use
  • Twilio numbers allow the assistant to make outbound or transfer calls to real people
  • If you use only Vapi phone numbers, transfer calls will not work, as they require Twilio integration for real call routing

Without Twilio, call transfers won't work.

6.2 How to Set Up and Link the Phone Number for the Assistant

  1. Go to the Phone Numbers module in Vapi
  2. Click Create a Phone Number
  3. Select Import Twilio
  4. Enter the Twilio phone number (created in your Twilio account)
  5. Enter your Twilio Account SID and Auth Token
  6. Give a label or name for the phone number for easy identification
  7. Click Import from Twilio
  8. Once imported, scroll to Inbound Settings
  9. Select the Assistant you want to link with this phone number
  10. Click Save

Test it: Make a test call to the Twilio phone number — the assistant should now answer and speak.

7. Call Logs — Know What's Happening

The purpose of the Call Log in Vapi is to record and track all calls made or received by your assistant.

It shows details like:

  • Caller number
  • Call time
  • Duration
  • Transcript
  • Recording

Perfect for:

  • Debugging issues
  • Performance reviews
  • Training improvements
  • Monitoring assistant quality

Why This Matters

Vapi isn't just another AI tool — it's an end-to-end voice automation framework. From voice recognition to real-time conversation to call transfers — all in one system.

It helps startups and teams:

  • Automate support
  • Qualify leads
  • Handle repetitive voice workflows
  • Maintain human-level quality

Final Thoughts

The future of AI assistants isn't chat-based — it's voice-based. And platforms like Vapi are making that future easier to build than ever.

Have you tried creating a voice AI yet? If not, maybe it's time to let your assistant pick up the next call.

Voice AI is no longer science fiction — it's a practical tool that can transform how businesses interact with customers, and Vapi makes it accessible to developers of all skill levels.