As above, you'd need a watch plan from one of the main networks which support it.
Other than that then your only option is using the 'call & text from other devices' function where, as long as your watch is connected to a WiFi network and your phone is also turned on then it will route calls / notifications from your phone to the watch. I've used it several times when I've been out on a night and knew the venue has free WiFi. Just left phone at home, logged into the WiFi at the place on watch and everything good. Of course, outside of WiFi then you'll have no connectivity.
