Consider the following scenario:
You run the Set-CsExternalAccessPolicy cmdlet to set the value of the EnableOutsideAccess parameter to false in Microsoft Skype for Business Server 2015.
Users are using Microsoft Skype for Business mobile apps and Microsoft Skype for Business on Mac clients.
In this scenario, Unified Communications Web API (UCWA) clients still can sign in from external networks even if the external access setting is disabled by the policy.
To fix this issue, install the December 2017 cumulative update 6.0.9319.510 for Skype for Business Server 2015, Web Components Server.
Note After you apply this update, run the following cmdlet to enable the external access:
Set-CsPlatformServiceSettings -EnableExternalAccessCheck $true