Hey there lwang9.
Nothing makes us happier than when we hear a customer wants to join the T-Mobile family! Sorry you're being stopped by that error. If you haven't already, you should Contact Us and have a Care rep take a look at this further. They have the tools to research this and help you get switched over.
Thanks, @tmo_mike_c. I tried to contact T-Mobile and the representative didn't know if my number could be switched to T-Mobile and she transferred my call to the tech department.
Finally, I don't want to "test" whether my number is portable manually.
What did the tech department tell you after you were transferred? Did they mention something about testing whether it could be ported or not?