Updated to iOS 11 can no longer FaceTime to 10.7

Hi folks

My mother has been using FaceTime to speak with my brother overseas from her 2007 MacBook running OSX 10.7 Lion to his iPhone 6s running iOS 10 with no issues. Recently he upgraded to iOS 11 and since then they cannot connect, regardless of who initiates the call.

Facetime rings, but when the call is accepted the recipient gets an error "connection failed". I tested from my iPhone 6, then running iOS 10 to both my brother's iPhone as well as to my mothers MacBook fine. I upgraded my iPhone to. iOS 11 and now I see the same issue that affected my mother and brother ("connection failed")

I suspect that there is an incompatibility between facetime on iOS 11 and OSX 10.7 and that it simply will not work but can anybody prove me wrong?