TMobile App & cell calls

I just switched to TMobile. My cellular and text all say they are using the “TMobile App” or a “travel line”.  What the heck is this?  There’s also a primary line, but it says, “No SIM,” which I know is not correct as I have a physical SIM card.  And I keep getting weird notifications, which is what lead me to find this strange configuration.  Anyone else getting this or know what this is?