well, seems quite logical that if you connect your telephone (which is a FXO, as you said) to the ATA FXS port and to the PSTN phone line, then both the PSTN phone line and the ATA FXS port provides the same protocol (analog in this case, but still a protocol). As so, you can use the same telephone set with both of them.
In other words, the PSTN line and the ATA port behaves in the same way to a telephone set that is connected to them. So, they must provide the same signals. I can, without loss of content, say that the PSTN phone line that reaches you delivers a FXS signal.
I thought it was this way before your question, but I just found some supporting documentation at patton.com/technotes/fxs_fxo.pdf
This documentation suggests clearly that I can call the PSTN line a FXS port. And that the port on your phone set is a FXO port.
I ask you that if you can point me to some other (better) documentation that states the opposite, please do, as I am still researching this content. Thanks.
I will quote the definition found there:
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. An FXS interface provides the following primary services to a subscriber device:
You may also see the FXS acronym rendered as Foreign eXchange System.
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. An FXO interface provides the following primary
service to the Telco network device:
on-hook/off-hook indication (loop closure)
How it Works
Because of the characteristics described above, a telecommunications line from an FXO portmust connect to an FXS port in order for the connection to work. Similarly, a line from an FXS
port must connect to an FXO port in order for the connection to work. When the FXO port on your
analog telephone is connected to the FXS port in the wall, you receive (FXS) service from the
telephone company and you hear a dial tone when you pick up the phone.