Apple watch carrier and iphone

It does matter.

The carrier of your iPhone needs to enable phone number sharing with your cellular Watch.

Note: Most budget carrier (MVNO) do not support Watch.

See carriers list here: Apple Watch - Carriers - Apple

Since the Apple Watch shares the iPhone's phone number, the carrier needs to support this feature, and you cannot have a separate carrier for the Apple Watch.

See the list of carriers posted by ckuan

I will also add that the GPS Apple Watch can do Everything the cellular model can do as long as your iPhone is nearby, where nearby is say somewhere in a 3-4 bedroom home.

Also if your cellular carrier supports WiFi calling, and you enable WiFi calling on your iPhone, then the GPS model can make calls via WiFi. The Apple Watch can also talk to your iPhone via WiFi when out of Bluetooth range, if both are on the same WiFi.