Requiring Phone Numbers for User Profile

As you set up your Organization, one option you have is to select whether or not you would like your users to be required to enter their phone number upon account creation. By default, this is not checked. If you would like to "Make phone number required on user profile" simply check the box here (image below). 

To get here, go to your Org Settings, General, and Preferences.

When a user creates an account, they will be taken to the screen below. If you have chosen to require a phone number, this field will be required to create an account.

This phone number will show on the member card.