On a lot of those Toshiba IPEdge systems, I used to get random lockups. I haven't seen services randomly restart. Maybe it restarted because of a SIP error. I am not sure.

A little feedback form the user's would be helpful. Did they see an error message on the display? All line busy or Call Abandoned? A call abandoned to me says that the problem was not the IPEdge.

I am not good at logs. I have a grasp on the voicemail logs. Maybe you can grab the logs from the messaging on that day to see if there was anything that looked odd, like the voicemail could not connect to the PBX. It won't identify the problem, but maybe it will tell you if the PBX is having a problem.