Can't sign in Skype for Business mobile apps after applying Skype for Business Server 2015 CU6 (December 2017 Cumulative Update)

After you apply Microsoft Skype for Business Server 2015 CU6 (December 2017 Cumulative Update), users are unable to sign in Skype for Business mobile apps over the Internet unless the following cmdlet is run:

Set-CsExternalAccessPolicy -EnableOutsideAccess $True

This issue occurs because the EnableOutsideAccess parameter in the ExternalAccessPolicy cmdlet applies to Skype for Business clients that use Unified Communications Web API (UCWA). For example, Skype for Business on Mac and Skype for Business mobile apps. The default value of the EnableOutsideAccess parameter is False that restricts external access for all UCWA clients of Skype for Business.

This update enforces the ExternalAccessPolicy only applies to Skype for Business desktop clients (Skype for Business on Windows and Skype for Business on Mac).

To fix this issue, install the following updates:

• March 2018 cumulative update 6.0.9319.516 for Skype for Business Server 2015, Web Components Server
• March 2018 cumulative update 6.0.9319.516 for Skype for Business Server 2015, Front End Server and Edge Server 
• March 2018 cumulative update 6.0.9319.516 for Skype for Business Server 2015 and Unified Communications Managed API 5.0 Runtime