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When this happens, does it recover on its own or do you need to do something to restore operation? If you do something, what?
"Ethernet connection lost" as a message on the phones could be a generic message that the phones can't communicate with their SIP gateway even though the physical link is up.
I would assume that you have one port on the switch configured as an access port on the VLAN used by the phones that connects to some type of SIP gateway, either an IP PBX, IAD, or firewall. Look at that link and the device connected to it for issues.
On the switch, you can do a show interface and look for errors and/or something that's half-duplex.
show int | i errors
show interface | i duplex
are a good quick look, if you see something out of the ordinary scroll through to find the specific interface in trouble.
clear counter will give you a clean slate to start from.