What is the interface type that you should use to connect a PSTN analog line to the VoIP network?

FXS (Foreign Exchange Station) provides battery voltage and dial tone to analog phones or fax machines - it connects to end devices, not to the PSTN.

FXO (Foreign Exchange Office) is the interface type that plugs into the PSTN analog line, acting as the device that connects to the central office side. It receives dial tone and ring voltage from the CO, making it the correct choice for terminating an analog PSTN line on a voice gateway. FXS ports, by contrast, provide those signals outward to analog endpoints.

E and M signaling is an analog interface used for tie-line connections between PBX systems, not for connecting to the PSTN subscriber loop.

Serial interfaces are used for WAN data connectivity such as Frame Relay or PPP, not for voice or PSTN analog line termination.