Apple today released a supplemental update to macOS Catalina 10.15.4. The update comes two weeks after releasing the initial macOS Catalina 10.15.4 update.
The update includes a bug fix for the FaceTime bug that caused Mac computers running macOS Catalina 10.15.4 not to be able to participate in FaceTime calls with devices running iOS 9.3.6 and earlier or OS X El Capitan 10.11.6 and earlier.
macOS Catalina 10.5.4 can be downloaded for free from the Mac App Store via the Update feature in the System Preferences app on compatible Macs. It can also be downloaded from the Apple Support site.
macOS Catalina 10.15.4 supplemental update improves the stability and security of your Mac.
Some features may not be available for all regions, or on all Apple devices. For detailed information about the security content of this update, please visit: https://support.apple.com/kb/HT201222