We have some Cisco 7935 and 7936 phones that seem to unregister from CM4.13 for no known reason (even during the middle of a call on some occasions). The phones do not recover from this situation unless they are unplugged from their power source.
Network logs show no issues (i.e. stp fluxuations or routing issues), and we do not have this problem with the 150+ other phones (7940,7960,7961g-ge's) that are connected to the same network switches on the same vlan
Additional information: The 4506's run WS-X4548-GB-RJ45V blades on 122-25.EWA13 and run the interface command "power inline never" on the ports where the 7936's are connected.
Below is a switchport configuration example where a 7936 is connected:
description 7936 Conference Phone
switchport access vlan 111
switchport mode access
power inline never