Discovered some more information that may be relevant. The 450 is being used with an Attendant Console softphone, I went back through the logs to look at previous calls. I noticed the problem call was the first call of the morning and that the previous call was a call answered in the evening when the phone is unattended. There was an incoming call, the ringer was set on and then the 450 sent an OffHook on it's own, there was no one there to answer the call. This "phantom" answering has been reported on a few occasions.