Ring Groups Intermittently Working

We have a DID routing to a ring group of three phones. Sometimes only two of the three ring (About 75% of the time actually).

-DID routes to ring group 2002
-2002 ring strategy is ringall
-Extension list is:

424 seems to be the one with the problem. The phone itself works fine. And again, sometimes it does ring with the group. Any ideas on what would cause it not to at other times?

Anybody have any input on this?

You would need to get a Asterisk CLI trace when it happens to know whats going on

Okay, great. Any tips or links to instructions on how to run that specific test?