changing my phone number

You can reach out to Customer Care via 611 from your T-Mobile device or try reaching out to the T-Force Team via Facebook or Twitter to have them take care of that for you.  There is also the random chat feature that appears and disappears on T-Mobile’s website as well.  You might be able to do it in your my.t-mobile.com account as well.  I personally have never looked in there for that option.