I am using iPhone 4 with iOS 6, and my boyfriend is using iPhone 5 with iOS 6 (recently upgraded to iOS 7).
Whenever I called him from my iPhone, it would say "Connecting..." when he picked up, and then after a few seconds it would say "Connection Lost". I tried a lot of times and it never worked.
Then I have my Mac setup with Facetime, and I was able to call his phone. And then I tried to use my iPhone to call my Mac (different ID), and it was connected. And then I tried to use my iPhone to call a friend's iPhone 4, and it also worked. So the problem is not on the iPhone.
So here's the summary:
- iPhone 4 calling iPhone 5 didn't work
- Mac calling iPhone 5 worked
- iPhone 4 calling another iPhone 4 worked
- iPhone 4 calling Mac worked