Purchasing a watch

Your iPhone and Apple Watch MUST use the same carrier. Make sure that you have an eligible cellular service plan with a supported carrier.

You van buy the waych direct from Apple or at many authorized Apple resellers (Best Buy, Verizon, etc.). To get started, see the "Which Apple Watch is right for you?" section in:

Buy Apple Watch Series 6 - Apple

The GPS only Apple Watch can do Everything the Cellular model can do AS LONG AS your iPhone is nearby, where nearby is generally somewhere in the same moderately sized home. The Cellular model allows you to leave your iPhone at home while you go for a walk, run, bike ride.

So if you keep your iPhone nearby most of the time, then maybe you do not need a cellular model. And the GPS only model does not depend on the cellular carrier, and the carrier does not know, nor care that you have a GPS only Apple Watch.

GPS

Apple Watch (GPS only) lets you send and receive text messages, answer phone calls and receive notifications when it’s connected to your iPhone via Bluetooth and Wi-Fi. The built-in GPS works independently of your iPhone for distance, pace and route mapping in workouts.

GPS + Cellular

Apple Watch (GPS + Cellular) has all the same features, as well as built-in cellular. It gives you the freedom to send and receive text messages, answer phone calls, receive notifications, and stream Apple Music and Apple Podcasts — even when you don’t have your iPhone with you.

What happens is you pay an extra monthly fee on your existing phone contract, the watch uses your existing phone number to make calls.In the US it's about $10 per month and about £10 in the UK,that is for the cellular version of the watch, if you buy the GPS only version of the watch you can make calls and send texts as long as you 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.So in reality your paying $10/£10 a month so that you can leave your phone at home.