There have been some issues where phone numbers in Outlook does not show in Lync 2013 Client. After some time we discovered that this was because of a Lync/Office update.  Yesterday Microsoft releast August update for Lync 2013 Client, this includes a fix for this issue. The updated version is 15.0.4641.1000

Can be downloaded from : http://support.microsoft.com/kb/2881070