How I Built an AI Phone Caller Using ESP32 and SIM800L

2024 filed under: reflections

Imagine an IoT device that can make real phone calls, play AI-generated voice messages, and trigger automatically based on events. That's exactly what I built using an ESP32 microcontroller, a SIM800L GSM module, and some creative audio integration. This project combines hardware, telephony, and AI in a fascinating way.

The Concept

The goal was simple but ambitious: create a device that can:

Applications could range from emergency alerts and appointment reminders to IoT notifications and automated customer service.

Hardware Components

ESP32 Development Board

The brain of the operation. The ESP32 provides:

SIM800L GSM/GPRS Module

This small module enables cellular communication:

ISD1820 Voice Recording Module

For audio playback during calls:

Other Components

Wiring the System

ESP32 to SIM800L

ESP32 TX (GPIO17) � SIM800L RX
ESP32 RX (GPIO16) � SIM800L TX
ESP32 GND � SIM800L GND
SIM800L VCC � 3.7V-4.2V power source (with 1000�F capacitor)

ISD1820 to SIM800L

ISD1820 SP+ � SIM800L MIC+
ISD1820 SP- � SIM800L MIC-
ISD1820 VCC � 5V
ISD1820 GND � Common GND
ISD1820 PLAYE � ESP32 GPIO (for trigger control)

Critical note: The SIM800L is extremely sensitive to power quality. Use a dedicated power supply capable of providing at least 2A, and add a large capacitor (1000�F or more) close to the module to prevent brownouts during transmission.

Programming the ESP32

AT Commands Basics

The SIM800L is controlled via AT commands sent over serial communication. Key commands for our project:

AT              // Test command
AT+CREG?        // Check network registration
AT+CSQ          // Check signal quality
ATD+91XXXXXXXXXX;  // Dial a number (replace with actual number)
ATH             // Hang up call
AT+CLVL=100     // Set speaker volume (0-100)

Sample ESP32 Code Structure

#include <HardwareSerial.h>

HardwareSerial sim800(1);

void setup() {
  Serial.begin(115200);
  sim800.begin(9600, SERIAL_8N1, 16, 17);  // RX, TX

  delay(3000);
  sendCommand("AT", 1000);
  sendCommand("AT+CREG?", 1000);
}

void sendCommand(String cmd, int timeout) {
  sim800.println(cmd);
  long int time = millis();
  while((time + timeout) > millis()) {
    while(sim800.available()) {
      Serial.write(sim800.read());
    }
  }
}

void makeCall(String number) {
  sendCommand("ATD" + number + ";", 1000);
  delay(20000);  // Call duration
  sendCommand("ATH", 1000);  // Hang up
}

Integrating Voice Playback

When the call connects, trigger the ISD1820 to play the recorded message:

void playMessage() {
  digitalWrite(ISD1820_PLAY_PIN, LOW);
  delay(100);
  digitalWrite(ISD1820_PLAY_PIN, HIGH);
}

void makeCallWithMessage(String number) {
  sendCommand("ATD" + number + ";", 1000);
  delay(5000);  // Wait for call to connect
  playMessage();  // Trigger audio playback
  delay(15000);  // Play duration
  sendCommand("ATH", 1000);  // Hang up
}

Voice Integration Options

Option 1: Pre-recorded Messages (Simple)

Record a message directly on the ISD1820 module:

  1. Press and hold the REC button
  2. Speak your message (up to 10 seconds)
  3. Release button to stop recording
  4. Press PLAY to test

Pros: Simple, works offline, no external dependencies

Cons: Fixed message, limited duration, manual recording required

Option 2: AI-Generated Voice (Advanced)

Use text-to-speech APIs to generate dynamic messages:

  1. Generate audio  Use OpenAI TTS, Google Cloud Text-to-Speech, or ElevenLabs API
  2. Store on SD card  Save MP3 files to microSD connected to ESP32
  3. Play via module  Use DFPlayer Mini MP3 module instead of ISD1820
  4. Trigger during call  Same principle as ISD1820

Pros: Dynamic messages, realistic voices, can include variables (name, time, data)

Cons: Requires internet initially, more complex hardware, API costs

DFPlayer Mini Integration

For AI-generated audio, replace ISD1820 with DFPlayer Mini:

ESP32 RX (GPIO4) � DFPlayer TX
ESP32 TX (GPIO2) � DFPlayer RX
DFPlayer SPK+ � SIM800L MIC+
DFPlayer SPK- � SIM800L MIC-

Use the DFRobotDFPlayerMini library to control playback programmatically.

Automation with n8n

To make this truly powerful, integrate with n8n (an automation platform) to trigger calls based on events:

Workflow Example: Emergency Alert System

  1. Sensor triggers  Temperature sensor detects fire (ESP32 sends HTTP request)
  2. n8n receives webhook  Processes the alert
  3. Generate voice message  n8n calls text-to-speech API with details
  4. Store audio  Saves to accessible location
  5. Trigger ESP32  Sends command to make call
  6. ESP32 calls number  Plays emergency message

Other Use Cases

Challenges and Solutions

Challenge 1: SIM800L Power Issues

Problem: Module keeps resetting, showing "UNDER-VOLTAGE WARNING"

Solution: Use dedicated 3.7V-4.2V power supply with at least 2A capacity. Add 1000�F capacitor across VCC and GND. Don't power from ESP32's 3.3V pin.

Challenge 2: Call Audio Quality

Problem: Voice message sounds distorted or too quiet

Solution: Adjust ISD1820/DFPlayer output volume. Use AT+CLVL command to set SIM800L speaker volume. Ensure proper ground connection between all modules.

Challenge 3: Network Registration

Problem: SIM800L can't connect to network

Solution: Check SIM card is activated and has calling enabled. Use AT+CREG? to check registration status. Ensure antenna is properly connected. Try different locations for better signal.

Challenge 4: Timing Issues

Problem: Voice plays before call connects or after it disconnects

Solution: Add sufficient delay after ATD command (5-10 seconds depending on network). Monitor call status with AT+CLCC command to detect connection.

Educational Value

This project taught me about:

GSM Module Operation

AI Voice APIs

IoT Integration

Legal and Ethical Considerations

Important: Before deploying an automated calling system:

This project is educational. Use responsibly and ethically.

Future Enhancements

Conclusion

Building an AI phone caller with ESP32 and SIM800L demonstrates how accessible IoT and telephony integration has become. What once required expensive enterprise equipment can now be built with $20 worth of components and some creative programming.

The combination of hardware (ESP32, SIM800L), audio (ISD1820 or DFPlayer), and automation (n8n, AI APIs) creates a powerful system for notifications, alerts, and communication.

Whether you're building an emergency alert system, appointment reminder, or just exploring the possibilities of IoT telephony, this project provides a solid foundation.

Start small, test thoroughly, and always use these capabilities responsibly. The technology is powerfuluse it to help, not to annoy.