I have had issues with receiving FaceTime calls, and thought it was a firewall issue or ISP issue. It was frustrating, only to stumble upon a completely different solution to my issue.

I was browsing the MacRumors forum trying to find a solution, and came across this post.

Like the original poster, I think it might have been something to do with my restore from my 3GS, and I followed these steps:

1. Turn off FaceTime (Settings, Phone, FaceTime OFF)
2. Turn on Airplane Mode (Settings, Airplane Mode ON)
3. Reboot Phone (Hold power button on top until red slider appears, swipe it, then once screen goes dark, press power button)
4. Turn off Airplane Mode (Settings, Airplane Mode OFF)
5. Turn on FaceTime (Settings, Phone, FaceTime ON)

Boom, I was able to receive FaceTime calls without any issues. :) It definitely wasn’t firewall-related. Whew. It’s real nice to receive a call on my iPhone and answer it in video! I can’t wait for this to work everywhere, not just WiFi. Makes me feel like I have a real phone, you know?