How do you check your phone number on t-mobile?

If you do not know your phone number, simply dial #NUM# (#686#) from your T-Mobile phone and a text message will be sent to your phone with your phone number. You can do this with or without available minutes because it does not cost anything to do. If you have minutes, another easy way to check would be to call your own home phone or someone's phone with caller ID and simply check the number that comes up.