At the moment, WhatsApp hides the phone number from people who do not have it saved when users set up a username. In a future update, users will be able to change this by choosing a different privacy visibility.
Some users may want to keep their phone number visible or add some exceptions.

WhatsApp is testing new phone number privacy for usernames | WABetaInfo
With the WhatsApp beta for Android 2.26.45.1 update from Google Play, WhatsApp is testing phone number privacy settings for usernames.
wabetainfo.com