FaceTime not working

Hi, I'm in the UK and my dad has gone to work in France. I've tried to FaceTime him but it just rings and rings. When I spoke to him on a normal call he said that it hadn't even rang on his phone. I don't know what to do he's connected to WiFi. He has tried ringing me on FaceTime but nothing happens at either end. I have an iPhone 5S and he has an iPhone 4 possibly a 4s.

Thanks in advance for any help you can give.

Gemma.