Watch Fall detection call via WiFi on nearby iPhone

You seem to have misunderstood what I posted.

THEKyd1 wrote:

Apple recommends you link your Apple Watch Series 8 GPS to your iPhone using WiFi; doesn’t say anything about Bluetooth being used.

WRONG. The default method of connection between any Apple Watch and its paired iPhone is Bluetooth. If you don't believe me, on your iPhone go into Settings/Bluetooth and see whether your watch is showing as Connected. Now go into the iPhone's WiFi settings and look to see what is connected, the only item is likely to be your WiFi router, which stays at home when you go out with the watch and iPhone.

(You might also find a direct wiFi connection from your watch to your router if you disable Bluetooth completely).

If I have Apple Watch Series 8 GPS (only; not GPS + cellular) and fall, it is supposed to contact my nearby linked iPhone; the iPhone will then contact emergency (911) via WiFi. My bottom line question is: if WiFi is not available for my iPhone to contact emergency, will it then switched to cellular to contact emergency?

I say again, I am not aware of any document which explains the routing priority for emergency calls from your iPhone, whether initiated from the paired watch or directly from the iPhone. HOWEVER good practice would give priority to cellular, because it has a better chance of having a good location fix. WiFi calling from your iPhone would be the backup route only if the iPhone's cellular connection fails.

To make it very clear to you, the Apple Watch (GPS models) emergency call functions route all calls through your iPhone. Apple designed emergency functions like fall our crash detection to be available anywhere, not only within your home. The default routing from your iPhone to the outside world is likely to be cellular, unless cellular fails and the phone has access to WiFi calling (which is likely only available within or nearby your home).

I don't remember seeing the routing priorities documented for emergency calls where both cellular and WiFi are available. However, the iPhone's preferred option should be cellular if it is available, because it brings the benefit of cell triangulation for the iPhone and increased probability for an accurate location. If it uses WiFi calling from the iPhone the location accuracy is going to be GPS only, which might or might not have a good fix particularly if the casualty event is indoors with poor view of satellites.

Note - The only way to prove this for sure would be a live test, which is likely to attract the wrong kind of official attention and annoy dispatchers.

As a point of interest, I would expect the communication between the Apple Watch and the iPhone will be Bluetooth rather than WiFi.