Unlocked iPhone with T-Mobile SIM
Go try to buy the phone now on the Apple site. It tells you when you're nearing checkout that what you're buying when you pay full price is an unlocked phone with the carriers Sim card
Every XR you pay full price for from Apple is unlocked.For the XS, you can select any carrier model, click continue, select activate later, and it lets you check out without entering any carrier info. They all say unlocked. So I wonder if you can buy any model and not just T-Mobile if you plan on using another carrier...
In the past I used to purchase T-Mobile version. Always unlocked and works with my AT&T sim. This year I was able to purchase a Verizon unlocked XS Max and it worked with my AT&T sim. To be clear, I purchased it from the apple store and not Verizon directly.