When I activated my iPhone, I received a brand new phone number. The first time someone asked me for it, I felt a mild embarrassment. "Oops," I realized. "I don't know my own phone number."
If you find yourself in this situation, don't panic. Your iPhone itself can tell you what your number is.
The first place to look is in your contacts list. Touch "Phone" then "Contacts." Scroll to the very top of the list and you'll see "My Number..."
Or, touch "Settings" and then "Phone." Your number is displayed at the top of the screen.