Apple today released a new support document which details issues related to making and receiving Facetime calls, a problem which started on April 16th, 2014. It appears the issue is directly related to a device certificate which expired on that day. As for the bug fix, users are directed to upgrade. If you are using an iOS 7 compatible device and still running iOS 6, your only option is to upgrade to iOS 7. Those running older iOS devices that are not compatible with iOS 7 are instructed to update to iOS 6.1.6.
As MacRumors points out, this effectively means that iOS 7 holdouts have no method of acquiring the fix for Facetime without making the jump from iOS 6 to iOS 7. When upgrading, iOS presents the most up to date version available. Most have already upgraded, with iOS 7 seeing an 80 percent adoption rate. Still, that doesn’t help those who have avoided the transition.
Some customers have avoided upgrading due to the aesthetics of iOS 7. After the initial release in September, 2013, there was a fair amount of backlash, with some customers looking for a way to downgrade. Despite the early growing pains, much of the dissent has subsided over time.
So if you have a newer iPhone or iPod touch and have experienced issues with Facetime, the only way to correct that problem is to jump on the iOS 7 bandwagon.