How did someone change my number on my phone without my permission?

Contact T-Mobile Support and have them look into the issue.  This is pretty much a peer help support forum for T-Mobile customers.  No one here has access to your account.  

Same thing happen to me. I was on the phone all day with customer service with no result. They said they would look into getting my number back to me and will contact me Feb 4th. Have you gotten anywhere with your problem? 

This happened to me as well. My phone number changed suddenly one morning. It is really scary since I didn't authorize this nor could I make any calls using the phone number. When I called T-Mobile they said I was calling from this new number but since I didn't set this account up, I didn't have the PIN associated with the account. I was stuck!

I got a welcome to T-Mobile text message with a brand new phone number. The number turns out was a pre-paid phone number with no pre-paid $s. T-Mobile couldn't figure out how this happened. I got my number back by going to the T-Mobile store. They were able to give me a new sim card with my old number.