they can operate as a trunk provider, or an extension, depending on what you’re trying to do.
i use a telular sx5e linked to a PAP2. to make a call to a mobile (i use t-mobile mates-rates for all the household mobiles) i can dial the extension setup for the FXS port on the PAP2, the sx5e then gives me a dialtone and i can then call the mobile … or …
i use the ‘speeddial’ i have setup, which then calls the extension, waits a few seconds for an answer, then plays DTMF to the sx5e, which calls etc etc.
beware of some of these units on eBay though, i’ve been bitten once and that was enough.