Hi Rick,
If the extension is a SPA phone I would suggest performing a factory reset on the phone (Settings button on the phone). Next, go to Status -> Support Tools -> Logs and enable logs for your SIP trunk. Ask the end user to note the time on the phone's clock and to contact you to capture the logs when the problem occurs again. This may help pinpoint where the problem is happening. Small Business Support can help look at the logs.
Chris