Open the Phone app
Tap Contacts
Tap My Card at the top
Your phone number will appear under your name
Open the Settings app
Tap Phone
Check My Number
Open the Contacts app
Tap your contact card
Find your phone number listed there
Open Messages
Start a new message
Tap the contact field
Your number may appear if your iPhone is set up with it
Open FaceTime
Your phone number may be shown in the settings or account section