The video calling glitch surfaced only when iOS 13.4 users tried to call those with iOS 9.3.5 or iOS 9.3.6.
The platforms are dated but that's the newest available iterations for a number of Apple devices, including the iPod touch (5th generation), iPad mini (1st generation), iPhone 4S, iPad (3rd generation), and iPad 2.
So, basically, the error prevented FaceTime with these specific devices.
@AppleSupport Hey Apple, since iOS 13.4 i can‘t call an iOS 9 Device over Facetime. That‘s a real Problem at these Days because it‘s the only way for me and my 2 childs to see our 90yrs. young Mother/Grandmother. Please fix. 🙏