Apple recently released iOS 13.4.1 to fix several bugs found in the previous software version. This includes the one affecting users’ FaceTime calls.
Devices with iOS 13.4 are currently unable to join FaceTime calls if the other end runs on iOS 9.3.6 and earlier or OS X El Capitan 10.11.6 and earlier. The new software update should be able to fix that so you can finally reconnect with your friends and family via FaceTime.
In addition to that, Apple is also fixing a Bluetooth issue in which it failed to function when users access from the quick menu. As for iPad users, some may have experienced an issue with turning on their Flashlight button at their Lock screen — these issues are all addressed in iOS and iPad 13.4.1.
The new update will be available for iPhone 6s and later models, iPad Air 2 and iPad mini 4 and their newer models and the iPod touch 7th generation.