We have run into this as well and support tells us "by design" you only see your direct phone number if you logon using phone number + RC password. All of our users either use SSO or they use their email address + RC password, so they are quite confused to see some unknown number + extension in the display rather than their own phone number. This is even worse for international users who see a US phone number plus extension.
The explanation I heard was that SSO (and logon by email) is tied to the users extension so it doesn't know about the user's direct phone number, while during logon with direct phone number it does. It may be oversimplification, but it would seem that the logon process could do a database query to see if the associated user object has a direct phone number and just pull that value in. As a poor man's option the ability to manually set what is shown as a user preference in the desktop app would be better than the current state.
I would like the ability to Hide Direct number on the Desktop/Mobile App. I have users with multiple desk phone and when their contact info they have all the phone numbers for the account and each phone associated to the user account. This is confusing and I will like the ability to hide all the numbers and only display 1 or two for each user account.