Open the Settings app
Tap Phone
Check My Number
Open Contacts and tap your name at the top
Tap your contact card to view your phone number
Open the Phone app
Tap Contacts
Find your own contact card
Check the number listed under your name
Go to Settings
Tap Messages
Check Send & Receive for your phone number
Go to Settings
Tap FaceTime
Check the phone number listed there
Dial a friend or another phone and see the number shown on their caller ID
Check the SIM card packaging or carrier account if the number is not shown on the iPhone