With the enhanced presence privacy mode enabled in a Microsoft Skype for Business Server 2015 environment, the call fails when you start a person-to-person or conference call to a user who uses Microsoft Skype for Business on Mac.
To fix this issue, install the March 2018 cumulative update 6.0.9319.516 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime.
Note After you install this update, run the following Skype for Business management PowerShell command to enable the feature for Unified Communications Web API (UCWA):
'Set-CsPlatformServiceSettings -AllowCallsFromNonContactsInPrivatePrivacyMode $true'
This feature can be set per pool, site, or global.