Does anyone have this problem; I have a 15'' MBP 2011 OSX 10.8.2., and an iPhone 4 running iOS 6.0.1. When my FaceTime receives a call, both devices are ringing just as they should, but one device does not stop ringing when I use the other to answer the call. However, the call is disconnected after some time (like, if you don't answer at all, the call attempt is dropped). So, my FaceTime calls are constantly dropped. IF I answer and then choose to reject the call with the other device, the call is disconnected.
This problem occurred some time ago, perhaps with the Mountain Lion upgrade, but not sure - I don't get that many FT-calls but from my family abroad.