I got a call from this number on my desk phone and also personal mobile number so I called them back. It is the main switch board for T-Mobile and the guy on the other end said he would not have any idea who called me. It could have been any TMO employee.
I received a call from this number. The lady claimed to be from T-mobile. When I asked what was the call regarding, she said she could not give me any information until I verified my information. Since I was not sure if she was actually from T-mobile, I did not verify any information. I called T-mobile's customer care number and they were unable to verify that this was actually one of their numbers. So, I called the 425-383-4000 and the guy stated it was the number to T-mobile's corporate headquarters switchboard.
This number called me on a Tuesday night at 6:30. I didn't call it back, but it makes sense that it is T-mobile. I filled out an online application with them about a month ago and it sounds like once they have your number they use it for other purposes.