Why do you have to do anything.
Have you activate number sharing on your Watch with your iPhone by your carrier.
Activate cellular service
You can activate cellular service on your Apple Watch during setup. If you don’t wish to, you can activate it later in the Apple Watch app on your iPhone. See Use Apple Watch with a cellular network.
Your iPhone and Apple Watch must use the same cellular carrier. However, if you set up an Apple Watch for someone in your Family Sharing group, that watch can use a cellular carrier different from the one used on the iPhone you manage it with.
Cellular service not available in all regions.