Basic question on PSTN vs PBX concept, FXS ports

[news2010a]

Hi, a question if I may:

Imagine I have a router configured with two FXS ports (as shown in the attached diagram). I know the respective router C2610-1 with (2) FXS ports can serve as a 'PBX' for my testing purposes because the FXS ports provide the dial tone. Since the PSTN network also provides the dial-tone, I can also say the same router named C2610-1 could be considered as my PSTN instead, correct? Am I overlooking anything in my statement?

[gogasca]

That will work definitly, this is what we sometimes use to emulate PSTN without Telco lines.

FXO-FXS when the FXS is our PSTN :)