Add cellular

Does your Apple Watch have a Red ring or dot on the Digital Crown? If it does, you have a cellular model, and you can contact your iPhone cellular provider about paying them more money per month to put your iPhone's phone number on your Apple Watch.

This assumes your iPhone cellular provider supports the Apple Watch.

Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

If you do not have a cellular model Apple Watch, then you will have to buy a different Apple Watch, as the GPS only model does not have a cellular radio chip inside.

If you purchased the non-Cellular version, you can not add that functionality.