How I Built an AI Phone Caller Using ESP32 and SIM800L (No Twilio)

How I Built an AI Phone Caller Using ESP32 and SIM800L (No Twilio)

Published: December 26, 2024
In this post, I’ll show you how I built my own **AI phone caller** using ESP32 and the SIM800L GSM module. This project can make real phone calls, play messages, and even use AI-generated responses – without any paid Twilio accounts. I built this because I wanted a free way to experiment with **AI + voice calling**. If you love IoT projects, you’ll enjoy this one.

What This AI Phone Caller Can Do

-> Make real phone calls using the SIM800L GSM module. -> Play a pre-recorded or AI-generated audio message during the call. -> Work fully offline (just a SIM card with balance). -> Trigger calls automatically or from a web interface.

What You’ll Need

1. **ESP32 board** (Wi-Fi + Bluetooth capable). 2. **SIM800L GSM module** (to handle calls and SMS). 3. A **working SIM card** (Jio, Airtel, or any other with call balance). 4. A small **speaker/ISD1820 voice module** (optional, for AI-generated voice). 5. Jumper wires, power supply (5V 2A recommended for SIM800L).

Step 1: Wiring ESP32 with SIM800L

Connect the SIM800L to the ESP32 as follows: -> ESP32 TX (GPIO 17) → SIM800L RX -> ESP32 RX (GPIO 16) → SIM800L TX -> GND → GND -> 5V → VCC (use a stable power supply!) Tip: SIM800L is sensitive to power drops, so use a dedicated power source.

Step 2: Test Calls Using AT Commands

Before adding AI, test basic calling. Upload this code to ESP32: 
#include <HardwareSerial.h>
HardwareSerial sim800(1);

void setup() {
  Serial.begin(115200);
  sim800.begin(9600, SERIAL_8N1, 16, 17); // RX=16, TX=17
  delay(3000);
  
  Serial.println("Dialing...");
  sim800.println("ATD+91xxxxxxxxxx;"); // Your phone number
}

void loop() {}
If wiring is correct, your phone will ring.

Step 3: Add AI-Generated Voice

I wanted the caller to play AI-generated messages. There are two ways: 1. Use **ISD1820 voice module** to record a static message. 2. Use **ESP32 + AI voice API (e.g., OpenAI TTS)** to generate audio on the fly and play via a DAC output. Here’s a sample text-to-speech conversion using OpenAI API:
curl https://api.openai.com/v1/audio/speech -H "Authorization: Bearer YOUR_API_KEY"
This generates an MP3 file from text, which you can store on ESP32 or SD card and play during the call.

Step 4: Trigger Calls from a Web Dashboard

To make it more fun, I connected ESP32 to **n8n automation platform** and created a small dashboard: -> When I type a phone number and message, n8n sends a trigger to ESP32 over Wi-Fi. -> ESP32 then calls that number using SIM800L and plays the message. You can even set it to auto-call daily reminders or emergency alerts.
🔗 Related: My AI Assistant Project – This shows how I used the same AI APIs for screen reading and answering.

Extra Features You Can Add

1. Record the recipient’s voice response and store it on SD card. 2. Use AI speech-to-text (Whisper API) to analyze what they said. 3. Schedule multiple calls automatically. 4. Add SMS support with AI replies.

Why This Project is Great

This project teaches you: -> How to use GSM modules like SIM800L for real-world tasks. -> How to integrate AI APIs for voice and text. -> How to trigger hardware from cloud automation tools like n8n. -> How to make IoT projects that are actually useful.

Final Words

If you want to build fun IoT projects without paying for Twilio or other services, this is perfect. 👉 Try this project and let me know your improvements. 👉 Also, check my Full Tech Journey blog to see how I started building AI + IoT projects. Thanks for reading! If you liked this, share it with your friends.

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