If activating cellular on an Apple Watch it can be done in one of 2 ways.
Either by pairing it to an iPhone, in which case it will then share its own service line with the watch, which of course requires the iPhone to have an active line it can share with the watch.
click here ➜ Set up cellular on Apple Watch - Apple Support
Or by setting it up using Family Setup. In this case the Apple Watch get's it's own cellular service line separate to the iPhone so whether the iPhone used to set it up has an active line or not, does not matter.
However this option limits the Apple Watch features.
click here ➜ Set up Apple Watch for a family member - Apple Support
You cannot setup a cellular Apple watch to a number of its own when pairing it with an iPhone, so yes the iPhone must have a phone line it can share with the Apple Watch.
1. battery replacement has been typically several hours in Apple Store and likely powered off?
Yes, the watch would loose call capability if its turned off. If what is getting its battery replaced is the iPhone, then no, the watch should not loose call capability while the iPhone's battery is being replaced.
2. SIM faulty but cannot be replaced until next operating hours of carrier?
Again, if this happens on the iPhone, the watch should continue to work. The watch does not support physical SIM cards, only eSims.
3. Damaged iPhone?
Again, no, if the phone is damaged, the watch can retain its call capability.
4. Battery depletion?
If the iPhone is out of battery, the watch can retain its call capability.
5. Powered off iPhone
Same as above.
The phone shares the service, but the watch does not then depend on the iPhone's functionality to retain call capability.
click here ➜ Use your Apple Watch without your iPhone nearby - Apple Support
