The Apple Watch shares the iPhone’s phone number. Your iPhone cellular carrier must support the Apple Watch and this phone number Sharing. The carrier charges a monthly fee for this service. In the U.S. the fee is $10 per month added to your iPhone cellular bill.
Here is a list of carriers that support the Apple Watch
https://www.apple.com/watch/cellular
Your iPhone carrier must be listed by name, and an NVMO carrier piggy backing on a carrier that supports the Apple Watch does not count.
The carriers that support the Apple Watch may require a specific subset of iPhone plans that will work with the Apple Watch.
Note: a GPS only model can do Everything the cellular model can do as long as your iPhone is nearby. Where nearby is say in the same 3-4 bedroom wood frame construction house.