Under Account, the number listed is your Voice number. If a number isn't shown, it means Voice isn't set up for the Google Account you're signed in to. If you want to add Voice to a personal account, tap Get a Google Voice number. If you want to add Voice to a work or school account, contact your administrator.
Are Google Voice numbers Anonymous?
When you make calls, you can hide your Google Voice number so that the people who you call can't see it.