When this issue occurs, Microsoft Lync 2013 only shows a user's telephone number in E164 format in a toast notification. This issue occurs when reverse number lookup (RNL) is invoked and you are enabled for the Enterprise Voice feature.
Note Lync 2013 will show the user's display name when you receive next calls from the user.
When you receive the first call, RNL is not completed and does not return a result although it is started.
To resolve this issue, install the May 12, 2015, security update for Microsoft Skype for Business. (KB link is tbd)
Note Lync 2013 was upgraded to Skype for Business in April 2015.