FXS cards are for connecting phones and faxes, FXO cards are for connecting incoming lines. You could connect a fax machine to an FXS, but not to an FXO.
As for ringing other POTS phones… I can’t comment on other countries, but generally speaking that won’t be a cake walk in the US. Most phone jacks in the US are wired up in a daisy-chain fashion, with one jack running the wire (and the line) to the next.
Some newer houses are using a star topology for phone lines, with each jack running back to a central patch panel in a wiring closet, but that’s more the exception than the rule. In that case, you could fairly easily rewire things so the telco line goes to the Asterisk box’s FXO and one of its FXS ports is distributed to the rest of the house.
With the traditional configuration, you could connect the Asterisk system’s FXS output to a phone jack if all the jacks were disconnected from the telco, but obviously then you have no phone service incoming to the Asterisk box. (One quick possibility: sometimes added-on extension outlets are wired back to the demarc box independently of the rest of the house. If that’s the case, you could conceivably house the Asterisk system at an independently-wired jack, disconnect the chained jacks from the demarc, and hook up an FXS to the now-unused telephone wiring.)
If you’re wired for ethernet, you could repurpose some of the ethernet wiring and outlets for telephones, or put all the phones on their own ATA. Beyond that, though, you’re looking at making great friends with Mr. Drywall Saw and Mr. Fish Tape.