Apple Watch Series 6 44mm

Assuming you pay T-Mobile to activate your Cellular Apple Watch

The Apple Watch shares the iPhone's phone number, and the carriers that support the Apple Watch charge a monthly fee. In the U.S. the fee is $10 per month. Here is a list of carriers that support the Apple Watch:

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

I have GPS only on T-Mobile and make calls from my Apple Watch, as long as my iPhone is nearby (which is almost always true). My Wife just got a Cellular Apple Watch and activated on T-Mobile today. We went for a walk (she left her iPhone at home, and I had my iPhone in my pocket), and we exchanged iMessages, made Cellular calls to each other, used Walkie-Talkie, etc... She was using the Cellular Apple Watch via T-Mobile, and I was using my Apple Watch via my iPhone with T-Mobile as my carrier.

So I've just provided you first had experience using T-Mobile with both a GPS only and a Cellular Apple Watch.

If it's a GPS only watch you can but you need to have your phone with you.If it's a cellular version you don't need to have your phone with you providing tmobile support the cellular watch.