Phone not ringing - intermittent

I have the same problem. I tried everything and finally I solved by putting the Vibrate mode on "Never".

@Mitasol
I had the same problem on my Galaxy S Android 2.2.1. Incoming calls would either just ring for 1 sec. before going silent or not ring at all. So unless I was looking on the phone I would miss the call.

I started to remove apps and check. After removing AndFire (firewall) the problem disappeared. I cannot guarantee that this was the one, but it looks most likely.

I will look for another firewall app to use.

Same problem with my ACE: sometimes it does not ring / nor vibrate 

I remember the problem appeared since the very first days of use.
The first time I noticed the problem I had already installed only Facebook and Messenger, nothing else.

I think there is something like an hardware problem, otherwise thousands of user would have experienced the same problem...

Now I have tried to choose "Never" for the Vibrate mode, as suggested in this topic, then I will try to uninstall Facebook...