I get this whenever I try to FaceTime my girlfriend in the UK. She has my old 5s running 8.x.x and I'm using an iPhone 4s running iOS 7.x.x and I am aware that there are issues using FaceTime between different iOS versions.
This happens when my wife FaceTimes me while I'm on certain apps doing things. If I'm not on an app, it goes through smooth. The worst are photo apps or photoshop apps. I need to send Apple feedback on this. Y'all send feedback too. https://www.apple.com/feedback/iphone.html#mn_p