Sorry for the delay in posting, I missed posting about the release of iOS 13.4 and now we already have an update to that with iOS 13.4.1. This update fixes a bug in FaceTime that iOS 13.4 introduced.
Basically the bug prevented some users on iOS 13.4 from participating in FaceTime calls with users running iOS 9.3.6 and earlier, as well as OS X El Capitan 10.11.6 and earlier.
Here is the official change log for iOS 13.4.1:
– Fixes an issue where devices running iOS 13.4 could not participate in ?FaceTime? calls with devices running iOS 9.3.6 and earlier or OS X El Capitan 10.11.6 and earlier.
– Addresses a bug with the Settings app where choosing a Bluetooth from the quick actions menu on the Home screen would fail.
As always you can find all of the iOS firmware files here: https://leimobile.com/firmware-downloads/