FXS - Foreign eXchange Subscriber interface (the plug on the wall) delivers POTS service from
the local phone company’s Central Office (CO) and must be connected to subscriber equipment
(telephones, modems, and fax machines). In other words an FXS interface points to the
subscriber

FXO - Foreign eXchange Office interface (the plug on the phone) receives POTS service,
typically from a Central Office of the Public Switched Telephone Network (PSTN). In other words
an FXO interface points to the Telco office

Cheers!