If you can be there when the problem is occuring, then monitor the line with a butt set and a digit grabber and see and hear what is actually happening on the FiOS lines when outdialing. My first thought, is like you, the problem should not be the FiOS lines. I had a similar problem on a Polycom system I removed and replaced with a real system. The POTS lines were Comcast, similar as they, also, are fake. The line was being seized, a ringback tone heard and then the call would drop. Comcast was there multiple times to repair the problem, replacing all their equipment, etc. to no avail. Before removing the Polycom, I just wanted to see if I could find it. The problem was the Polycom was inserting an "A" DTMF (outband) in the outdial string, randomly. This caused the CO to direct the call to some service that was anticipating more out of band signals. When the service did not receive the signals, it just disconnected. There was never a busy or fast busy or any indication that there was something wrong.
I asked a Verizon tech if he knew what that service might be and he mumbled something about Autovon, but I think he was clueless.
So, before thinking this is a dead end, monitor and see if you can detect something.
Rcaman
