Thanks to Dan Balitewicz for the update on this one.
Microsoft is implementing Microsoft Identity Platform 2.0 (an evolution of the Azure Active Directory identity service) which uses the OAuth 2.0 authorization protocol. If you have SfB Certified IP Phones (3PIP) from AudioCodes, Crestron, Polycom or Yealink signing into to Skype for Business Online (or Microsoft Teams via cloud interop), you will need to firmware update them and take one time tenant admin steps to approve each vendor’s phones sign in to your Office 365 tenant (once per vendor).
As is becoming the trend with Office 365 change deadlines, this date has shifted from July 1st 2019 to January 15, 2020 and now to July 15th, 2020,
Microsoft updated their blog with the new deadline here.
You can find the details on the update on my previous blog here.