I'm not sure when it stopped working, but it used to be that you could go into Outlook and in the Search People box, you could type in a phone number that was in AD and it would do a lookup from the GAL and show you who the number belonged to.
We populate the mobile number field in AD and it syncs via AAD Connect. You can search someone's name in the Search People box and it shows the contact info. We just can't do the reverse lookup anymore. It was super handy and I wish this functionality was brought back.
Mobile number is displayed when searching by a person's name