Hi,
Firstly, get a very clear description of the problem from the end user.
Then check the physicals. Ethernet disconnected can be a problem with the cabling. Not sure why it would be time related, could just be intermittent. Otherwise maybe someone is sharing the cable, and some device is interfering.
If you have access to the switch. Get the mac address, locate the phone port via sh cdp neighbor and check the logs.
Is the port really going up and down etc. Time stamps should help you verify the end users story.
Cheers,
Tim