Microsoft are moving all online services to Transport Layer Security (TLS) 1.2+. As a result, Microsoft will be removing support for TLS versions less than 1.2 from their online services, beginning October 31, 2018.
Starting on October 31, 2018, all client-server, server-server and browser-server combinations must use TLS 1.2 or later protocol versions to be able to connect without issues to Skype for Business Online. This may require certain client-server, server-server and browser-server combinations to be updated, and if no action is taken to upgrade to TLS 1.2+, then your Skype for Business Online service may be affected.
If you are using any on-premises infrastructure for hybrid scenarios or Active Directory Federation Services, make sure that these infrastructures can support both inbound and outbound connections that use TLS 1.2.
Prior to October 31, 2018, you will need to prepare your environments for the upcoming TLS 1.2 change to maintain reliable Skype for Business services.
Please click here for documentation covering scenarios for Lync/Skype client connectivity to Office 365, On-premises server integration with Office 365 and 3rd Party integration with Skype for Business Online including detailed remediation guidance.