Not everyone who works at a cell phone store is 100% knowledgeable on all aspects of the phones and services the company offers.
BTW, you should be aware that the 6T that T-Mobile sells is "customized" (crippled) for T-Mobile and has different features than the regular phone, (and is locked).
Interesting. The Oneplus 6T is "crippled". Interesting comment. The phone has only one Sim slot ... like most US phones do. The bootloader is locked...just like most US Phones are. I think you are doing people a disservice by using the unfair categorization of crippled of this phone from the first Carrier selling this phone. People have the option of going to the Oneplus website and buying their phones with dual sim slots and the bootloader isn't locked and that is what people should be told. I got a good trade in deal with T-Mobile on this Oneplus 6T...and despite it being "crippled" I am very happy with the phone and the trade in deal. Without the promotion ran by T-Mobile I probably wouldn't have gotten this phone and don't feel that it is crippled at all.