I don't think this will be of too much help for most people, but I'll describe my situation for general info. I'm using the watch on Telus in Canada, and only have a sim in the watch, and my phone is sim-less so no cell service or number assigned to it, just wifi for internet. If I disable bluetooth connection between the devices, they still remained synced as the phone has wifi, and Ok Google still works on the watch. When I dial through Ok Google it automatically selects the watch number, I suppose as there is no other option for it, but there is no prompt or issue.
Oddly though, when I try to send a text with OK Google using the stock messenger app it auto defaults to try to send via the phone, which it can't, and errors out. I can still go into my contacts on the watch and send a text by selecting watch though. If anybody knows a way to set this to always send by watch that would be amazing.