That's your own phone number. You can go to settings and look for your Sim card to edit your number there. I don't know how to answer you if you are Sim free.
Don't understand why you should have to enter you r own number? When I'm home, it says my number. I never entered it. When I've used foreign sim's it never shows my number. So what's the point if you have to manually enter your phone number???
You all are too smart lol. I have lots of friends that don't know their phone number when you ask them. It's also useful for showing people when they ask for your phone number. Instead of repeating a string of numbers 2 or 3 times, just hand them your phone.