@PATMOY ,
You could execute the following scripts in Skype for Business back end server to get the LastLogIn Time of users:
SELECT UserStatistics.UserId, Users.UserUri, UserStatistics.LastLogInTime
From [LcsCDR].[dbo].[UserStatistics]
Inner JOIN [LcsCDR].[dbo].[Users] ON [LcsCDR].[dbo].[Users].UserId = [LcsCDR].[dbo].[UserStatistics].UserId
WHERE LastLogInTime IS NOT NULL
ORDER BY [LcsCDR].[dbo].[UserStatistics].LastLogInTime Desc
If the user never log in Skype for Business client, it will show no record.
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