I have a SIM808 module connected to an ESP8266. RX and TX are wired up, but I’m not getting any response from the SIM808—it appears disconnected.

Could this be due to insufficient current?

I’m supplying 3.8V to the Li-ion pin from an external source, with a shared ground between the ESP8266 and SIM808. Using GPIO 1 (Tx) and GPIO 3 (Rx) for serial communication.

Any troubleshooting tips?

 

Welcome to AAC.

The maximum rating for \(\mathsf{V_H}\)—the logical high input to the digital pins on the SIM808 is 3.1V—including the GPIO and UART. You make have fried the RX pin if you didn’t use a resistor.