Any cellular apple watch uses your existing phone number by means of an E-sim that is built into the watch,the downside is that you must use a network provider that supports the cellular watch,here is the list;
Carriers that support the apple watch.
An alternative is to buy the GPS only version of the watch,this makes calls and sends text through any network provider,the only proviso is that you must have your phone with you .
With the cellular version of the watch you can make calls and send texts without having your phone with you.
The cellular version costs $8 per month in the US on top of your monthly contract.