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

FXS (Foreign Exchange Station) provides dial tone and loop current to analog endpoints such as phones or fax machines - it connects to subscriber devices, not to PSTN lines.

An FXO interface connects to the PSTN analog line coming from the central office (CO), as it is designed to receive the dial tone and loop current provided by the telephone company. On a VoIP gateway, the FXO port acts as an analog phone would on the PSTN side, allowing the gateway to bridge PSTN calls into the VoIP network.

E and M (Ear and Mouth) interfaces are used for trunk-side connections between PBX systems or between a PBX and a carrier using analog tie lines, not for standard PSTN subscriber analog lines.

Serial interfaces are used for WAN data connections (such as T1/E1 with HDLC or PPP) and are not applicable to analog voice signaling from the PSTN.