Consider the following scenario:
You create a contact in Microsoft Outlook with only a name and a phone number.
You search this contact in Skype for Business 2016 and add it to the contact list.
You restart the Skype for Business 2016 client.
After you restart the client, you may find the contact name is displayed as the phone number instead of the contact's name.
To fix this issue, install the December 4, 2018, update (KB4461545) for Skype for Business 2016.