Mid Thursday, Apple issued a support document that says for people who can upgrade to iOS 7 should do so as soon as possible, for FaceTime to work again.
The document says in part:
“If you started to have issues making or receiving FaceTime calls after April 16, 2014, your device or your friend’s device may have encountered a bug resulting from a device certificate that expired on that date. Updating both devices to the latest software will resolve this issue.
If you’re using iOS 7.0.4 or later or iOS 6.1.6, this issue doesn’t affect you.
If you’re using these versions of OS X or FaceTime for Mac, this issue doesn’t affect you:
– OS X Mavericks v10.9.2 or later
– OS X Mountain Lion v10.8 with the latest security updates
– OS X Lion v10.7 with the latest security updates
– FaceTime for Mac version 1.0.5 or later for Mac OS X v10.”
Apple’s FaceTime issues first cropped up on April 16th.