I was on another support thread before I found this one and saw a post that explains what's going on. It has nothing to do with a generational model - iPhone4, 4S, 5, etc.
Here's the deal - Apparently, Apple lost some patent suit with a company that covers some of the methods for FaceTime and iMessage. The guy that posted actually attended the court hearing, and listened to Apple's attorney explain that to avoid paying any royalties and infringing on these patents, Apple decided to change how FaceTime connects with other users.
According to Apple's attorney, FaceTime calls used to be a direct connection. Now Apple has shifted ALL FaceTime calls onto Akamai's relay servers for data packet movements. Relay servers are DINOSAURS when it comes to next generation, real-time communication via session intitiation protocol (also called SIP). This is why the continuity is so crappy lately.
I'm sorry, but it looks like everyone's FaceTime calls are going to be sketchy at best for now on. Or, at least until they make a decision to pay for the technology that Apple was found guilty of infringing upon. How can a company that has so much money throw their customers under the bus like this? I suppose that's how they get so rich...screwing the customer over if they have to.