As above, the documentation may help.
But this heavily depends on the carrier and if you have WiFi Calling *and* WiFi messaging supported by the carrier and supported by the carrier on connected devices.
1 - Your watch can make a phone call via the direct link to your iPhone by default.
2- With some settings, your other devices (iPad and watch etc) can make calls via the same wifi network as your iPhone but your iPhone makes the phone call
3 - With some carriers, your iPhone is able to make calls over WiFi instead of cellular, and some carriers offer SMS that way too. With those carriers, only some also allow this to be used by a connected device like a watch and if your carrier supports that then the watch should be able to make the call via wifi calling if your phone dies and you are on wifi. But I've never been able to test that functionality since my carrier does not support WiFi calling on connected devices, only on the iPhone, and no messages either.
It sounds like you are asking about option 2 which is very easy to set up. Unless by not nearby you mean completely not on the same wifi and possibly off, which option 3 is what you need and I've never seen anyone (but in the UK) have that set up.