Apple Watch 4

Maybe this is what you are looking for

https://support.apple.com/en-us/HT205547

Footnote #2

If you're on Wi-Fi and want to call a phone number, make sure that Wi-Fi calling is turned on for your Apple Watch. Otherwise, try making a FaceTime audio call.

That is not undocumented. Please read the following:

https://support.apple.com/en-us/HT204562

I have used my GPS ONLY Apple Watch to make/receive phone calls when I'm away from home and not connected to WiFi.

If the phone and the watch are on a wi-fi network, are setup for wi-fi calls and only if your cellular carrier allows wi-fi calling.

ASC - Late 2012 Mac mini, macOS 10.14.5; Watch, series 3 cellular, watchOS 5.2.1; iPad Air 2 & iPhone 8+, iOS 12.3.2; TV 4th Gen, tvOS 12.3;  HomePods, audioOS 12.3 and Airport Express w/ Airplay 2

I disagree slightly. If the iPhone and watch are linked by Bluetooth and the phone has a cellular signal WiFi calling is irrelevant, the watch would behave like a remote bluetooth handset on the phone’s regular connection. WiFi calling for the watch only becomes a requirement if the iPhone does not have a cellular signal, or the phone is out of range from the watch but it can still see wifi.

Not so much what is documented, more what is not documented as a requirement for either Watch or iPhone. As far as I can see the iPhone requires a cellular service for voice, and if the plan also provides data it will be usable. There is no documented requirement for WiFi calling on the plan, but if available it can be used, and it would be unusual for a plan not to offer WiFi calling at least here in UK. Apple Watch GPS will connect over Bluetooth to the paired iPhone and will be fully functional, again no mention of WiFi calling as a mandatory requirement.

Practical observation - Apple Watch GPS will connect through the iPhone from locations where WiFi is not available, so WiFi calling is not an available routing and the phone must be passing the watch traffic over cellular, using either a voice or a data circuit as appropriate to the type of traffic. Calls can be switched between phone and watch in these conditions so that would imply a voice circuit is being used.

Further investigation verifies that you are correct. A non-cellular Watch can make a call over its paired iPhone using BT if it is within BT range of the phone.

Which has nothing to do with the wi-fi calling feature of the Watch.

I stand corrected..

We have established an undocumented use of the Watch, whereby the GPS-only version or the cellular version with cellular turned off, can make calls through the BT paired iPhone, even when there is no wi-fi network available.

As Branta_UK stated, the watch appears to be functioning in the same manner as a BT remote handset to the watch.

Yes.

Is this a documented ability of the Watch (where is this explained in the user guide) or something you guess to be true?

Sorry, that sounds like a lot of mumbo jumbo that you made up.