Installing the trusted root certificate
Applies to: Lync Server 2013 | Skype for Business 2015 Installing a trusted root certificate is necessary only if you are notified that the certificate of authority is not trusted on any machine. This can occur when you use a private or custom certificate server instead of acquiring certificates from an established public certificate of authority.
https://docs.microsoft.com/en-us/skype-sdk/sdn/articles/installing-the-trusted-root-certificate
