Assume that a Microsoft Skype for Business Server 2015 user only signs in a Skype for Business mobile app on which the advanced privacy mode is enabled. A Public Switched Telephone Network (PSTN) or Skype for Business caller who isn't in the user's contact list calls the user. In this situation, the call fails.
This issue occurs because Unified Communications Managed API (UCMA) doesn't let the incoming call to go through if the called mobile client is in privacy mode.
Article ID: 3203150 - Last Review: Nov 11, 2016 - Revision: 1
Skype for Business Server 2015