Is any way to know how many days have elapsed since a phone passed from "unregistered" to "None" status? Or any way to know how many days have the phone been in "unregistered" status?
Im using CUCM 10.5.2
You can try collecting Event Viewer and App Sys logs but it will give you the output based on logging partition. CUCM doesn't have any specific logs or counter to display the time number of days phone went into unregistered state.
You can only view the event for the phone went into unregistered state after pulling the event viewer app logs if cucm didn't purge the logs and if you can find the phone MAC in logs and you can figure out the time for unregistered state.
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I believe in a 48-72 hour window an unregistered phones moves into an unknown status or instantly just with a CUCM restart or process restart
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Depend on the value from a RIS service parameter, default is 3 days
RIS Unused Cisco CallManager Device Store Period
Aside from that, you would need to look at CUCM traces to find out the last keepalive from the phone to find that out, along with the value you see and the value from the above parameter.
CUCM 12.0 does provide the last time it was registered and last time it was active under the device page.