Which port is configured for connection to the PSTN?

T1 is a digital interface capable of carrying voice via PRI or CAS signaling; it is a separate technology category and not the analog port type specifically associated with PSTN connection in this context.

FAS is not a recognized standard voice gateway port designation and is not a valid interface type for any telephony connection.

FXS (Foreign Exchange Station) provides dial tone and ring voltage to subscriber analog endpoints such as phones and fax machines, so it connects to user devices rather than to the PSTN.

FXO (Foreign Exchange Office) is the port type specifically designed to interface with PSTN lines by connecting to a Central Office or wall jack. The FXO port receives line power and dial tone from the telephone company, making it the correct interface for PSTN connectivity on an analog voice gateway or router.