During my CVOICE recent training, my instructor stated:
"Think that the FXS is the port connected to the wall. FXO is the port connected to the phone".
Then later I thought more about it and I've seen topologies in which the FXS port (which I know is the port which provides the dial tone) is connected directly to a (analog) phone. Then the FXO port from a gateway/router is connected to a PBX.
Do you think the instructor's remarks on FXS and FXO make sense? Below are the definitions of FXS and FXO by cisco.com.
FXO?The Foreign Exchange Office interface allows a connection to be directed to the PSTN's central office. The FXO interface also allows a connection to be directed to a standard PBX interface or if the local telecommunications authority permits. The FXO interface can also be connected to analog PBX station lines. This interface is of value for Off-Premises eXtension (OPX) applications.
FXS?The Foreign Exchange Station interface allows connection for basic telephone equipment, and keysets, and supplies ring, voltage, and dial tone. The FXS interface can be connected to standard analog PBX central office/network trunk ports.
E&M?The recEive and transMit interface allows connection for PBX trunk lines (tie lines). E&M is a signaling technique for two-wire and four-wire telephone and trunk interfaces.
what your instructor has said is correct though could have been worded better,
The FXS acts as the line (port on the wall) so you would plug a phone into it.
The FXO acts as a phone so you would plug a line into it
I hope this helps