Fall detection

The following webpage may help you understand what your Apple Watch can do when your iPhone is not nearby

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

With respect to your question, if your iPhone carrier supports WiFi Calling, and you have this enabled on your iPhone, then if your Apple Watch is on WiFi, it can still make a phone call. This is easy to test, you turning off your iPhone and calling you home landline phone, or a friend from your Apple Watch. An emergency 911 call is no different from a regular call.

Within 30 feet or so unless you have a Cellular Watch and plan.