Rephrasing.
If you pay for cellular service on the Apple Watch Ultra, then the Apple Watch Ultra can make phone calls without the iPhone. Getting cellular service for the Apple Watch Ultra involves getting an add-on plan with your iPhone carrier. Typically this is $10 (U.S.) for this iPhone add-on cellular plan.
If you do not get an add-on Apple Watch Ultra cellular plan, OR if you are in an area with weak cellular signals, and the Apple Watch Ultra cannot see a cellular tower, BUT the iPhone, with its larger cellular antenna does see a cellular tower, then the Apple Watch Ultra will piggy-back off of the iPhone, and make the call using the iPhone, if and only if you happen to have your iPhone with you at the time.
WiFi calling typically only happens when you are at home (not always, but mostly), and so if you happen to crash in your driveway or garage, then the Apple Watch Ultra could also use WiFi to make the emergency call.