Build an ESP32 GSM Calling Device with Auto Answer Using GeoLinker

This project creates a portable wireless voice communication device by integrating an ESP32 microcontroller with a GeoLinker GL868 cellular development board. The ESP32 DevKit v1 acts as the central processing unit, running code written via the Arduino IDE and utilizing the specific GL868_ESP32 Arduino library to command the cellular module. For power, the system relies on a 3.7V lithium battery connected via a JST connector, ensuring stable current delivery during high-power cellular network operations. Voice input and output are handled by an analog condenser microphone and a 4-ohm speaker respectively, converting acoustic sound waves into electrical audio signals and vice-versa through the GL868's audio interface. A push button is wired to the ESP32 to trigger manual outgoing phone calls. Meanwhile, the software constantly monitors the cellular module for incoming calls, utilizing an auto-answer routine to pick up incoming connections automatically without manual intervention. By combining standard microcontroller digital inputs with cellular AT commands processed through the serial or library interface, the device bridges local user inputs and audio transducers with a global GSM network, utilizing standard 2G SIM cards for cellular connectivity and mobile network registration.