How to Build an AI Phone Agent That Answers Customer Calls 24/7

Deploying an interactive voice agent requires orchestrating a fast, real-time pipeline. Any latency over 1 second during conversations destroys user experience, making the agent feel robotic and laggy.

This guide provides a step-by-step developer tutorial showing how to connect Twilio (telephony), Vapi/Retell (voice orchestration), ElevenLabs (voice generation), and Weavecode API (LLM brain) to handle customer support calls autonomously.

1. System Integration Flow

  Customer Call ──► Twilio Phone Number
                           │
                           ▼
                     [ Retell/Vapi ] ◄──► [ ElevenLabs TTS ]
                           │ (Stream Audio)
                           ▼
                   Weavecode API (Fast LLM Brain)

1. Twilio: Provisions the phone number and handles routing.
2. Retell AI / Vapi: Orchestrates the incoming call, manages Speech-to-Text (STT) transcription, handles customer interruption logic, and streams outputs.
3. Weavecode API: Processes the transcription and generates immediate conversational text.

2. API Implementation Guide

We will create a FastAPI backend endpoint that handles Retell AI's dynamic system webhook callbacks.

Step 2.1: Implement the FastAPI Server

Install FastAPI and dependencies:

pip install fastapi uvicorn openai

Step 2.2: Build the Callback Server (main.py)

from fastapi import FastAPI, Request
from openai import OpenAI
import os
 
app = FastAPI()
 
# Initialize Weavecode OpenAI-compatible client
client = OpenAI(
    base_url="https://api.weavecode.ai/v1",
    api_key=os.getenv("WEAVECODE_KEY")
)
 
@app.post("/voice/callback")
async def voice_callback(request: Request):
    payload = await request.json()
    transcript = payload.get("transcript", "")
    
    # 1. Evaluate user transcript using high-speed model
    response = client.chat.completions.create(
        model="google/gemini-1.5-flash",
        messages=[
            {"role": "system", "content": "You are a customer support agent. Give short, direct answers under 30 words."},
            {"role": "user", "content": transcript}
        ]
    )
    
    answer = response.choices[0].message.content
    
    # 2. Return text output to Vapi/Retell to convert to speech stream
    return {
        "response_text": answer,
        "actions": []
    }
 
if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=8000)

3. Configuring Telephony (Twilio integration)

1. Navigate to Retell AI Dashboard → Phone Numbers → Buy/Import Number.
2. Connect your Twilio Account SID and Auth Token.
3. Set the Inbound Webhook URL to your deployed FastAPI callback endpoint: https://your-server.com/voice/callback.
4. Call the phone number to start interacting with your autonomous agent.