Cellular Apple watch

As fireyjack says, if you want a cellular Apple Watch, your iPhone needs to be using a carrier that supports the Apple Watch.

A GPS only Apple Watch can do everything the cellular model can do as long as your iPhone is nearby.

No but your present network provider must support the cellular watch,here is a list of providers that support the watch.

Apple Watch - Carriers

Apple watch has a built in Esim that uses it's paired phone's number to make calls.