Cisco IP Phone 7911: Error "Ethernet Disconnected"?

The phone is connected to a Cisco 3750 switch port with PoE enabled. The connected port is on the voice VLAN. CAT5 cable has been tested end-to-end and meets 568B. Port speed/duplex is 100/full. RJ-45 connectors were replaced at both ends, twice. No errors on interface. But I still can't seem to resolve this issue. The phone registers with Call Manager in a test enviroment, but not when deployed. Is there something I'm missing?