The Apple Watch shares the iPhone’s number and the carriers that support the Apple Watch charge a monthly fee of approximately $10 U.S., which is added to the iPhone’s bill.
if the iPhone’s carrier does not support the Apple Watch, then you either change carriers, or use the Apple Watch as in GPS only mode.
A GPS only Apple Watch (or a cellular model with no cellular contact), will do EVERYTHING the activated cellular model will do AS LONG AS the iPhone is nearby. Where nearby is in the same moderately sized wood frame constructed home.
The only other option is to treat the Apple Watch as it is being used by a dependent child, which is controlled by the parent’s iPhone. In this mode the Apple Watch functionality is degraded and many features do not work.
Apple Watch - Carriers - Apple