Just to add a data point … I was entirely phone-less for about a week after I idiotically put my beloved Xs through the wash. The phone wasn’t merely off; it was completely dead. In the interim, I used my Ultra exclusively. (I’m on T-Mobile in the States.) The only things that didn’t work were SMS and a couple third-party apps which are nothing but Bluetooth-connected frontends to corresponding phone apps.
SMS is expected; the watch doesn’t speak SMS (for whatever incomprehensible reason) and instead uses your phone as a relay … so long as the phone can send and receive SMS, and there’s some sort of a network connection between the phone and the watch (even if on opposite sides of the planet), SMS will work. And iMessage just requires some sort of Internet connection between the watch and Apple’s servers.
If you have a cellular-enabled watch and can’t make and receive phone calls on the watch regardless of the state of the paired phone, there’s something worng. Maybe with the watch, maybe with the carrier … maybe a reboot will fix it, maybe the carrier needs to do something, maybe there’s physical damage, maybe it’s something else … but it’s not only supposed to work, it’s a major selling point, the entire reason why you pay more for cellular-enabled versions of the watch. Like buying a car with anti-lock brakes but still having to pump them to keep them from locking up. (Except I suppose all cars sold today have anti-lock brakes … so get off my lawn!)
b&