Apple Watch

You can buy a cellular Apple Watch and then NOT sign up for cellular service with your iPhone carrier. But then why not buy a GPS only model. Unless you want the Stainless Steel model, then that only comes in a cellular model, but you can ignore the cellular and use as a GPS only model.

If you mean get cellular service on an Apple Watch WITHOUT paying a carrier for the service, then no, you cannot do that.

NOTE: If you always have your iPhone nearby, you can do Everything on a GPS only Apple Watch that you can do on the cellular. The GPS only Apple Watch will just use Low Energy Bluetooth to get the the iPhone and use its radios.

You may find the following 2 articles useful

https://support.apple.com/en-us/HT205547

https://www.imore.com/heres-what-apple-watch-can-do-without-iphone

And if you decide to get a Cellular model, this is a list of carriers that support your iPhone and the Apple Watch, along with the models numbers, countries they operating in, and the cellular frequencies each model uses:

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

It is combined with your telephone service. I pay AT&T $10/month plus applicable taxes.