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

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.