Hello, I inherited a rather old batch of Cisco gear and just ran into this very same problem. I had a Cisco 7942G phone, just bought, and plugged it into a network run that I just had fixed. The phone works fine when I plug it into my testing switch, a Catalyst 3560 PoE-48 but when I plugged it into the line where it should be staged, linked to a Catalyst 3560 24-PS, it gave me this exact error "Ethernet Disconnected". I tested the line with my Fluke LinkRunner and the circuit was perfect. I plugged in my Macbook at the staging area and everything worked perfectly. The phone would just not register at all. For me the fix was to login to the 3560 24-PS switch and look at the port that I had plugged it into. Fa0/24 was set to "no mdix auto". When I cleared the setting and saved the config, everything then worked perfectly. The phone registered and everything. As I said, I inherited a network that is mostly wreckage. It's getting better, slowly. Very slowly. :) HTH.
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