Yealink phones have recently been updated to automatically include internal company contacts in the phone's remote directory.  These contacts can also include the cell phone number for internal extensions if it has been populated via the User Portal under User Settings.

This directory will appear by default, and update automatically for any changes to the account daily, and on every fresh interaction with the search function.  It can be found on the phone itself under Menu \ Directory \ Remote Phone Book.  By default, the Directory key on the home screen will take users to the Local Contact Directory stored on the phone, not the Remote Phone Book this article refers to.  

If desired, the key can be re-mapped to the Remote Phone Book by modifying the Programmable Key type from Directory to XML Directory for faster access.  This can only be done in the web interface of teh phone itself currenlty, Under the DSS Key tab \ Programmable Keys.