Start by accessing the app's Settings by accessing the dedicated section on iOS, and by unfolding the app's main menu on Android. Then access the Privacy and Security options.
Then go to the Phone Number menu, and change the Who can see my number option. Depending on the privacy you want, choose My contacts, or Nobody if you don't want your number to be visible to any user.
You should choose one of these two options if you want to use the application incognito, especially if you participate in public discussion groups. It will be possible, if necessary, to add exceptions to this rule. You can indicate that the number is never visible for some contacts, or on the contrary that it is always visible for others.