You have Skype for business 2015\2016 or Lync 2013. Everything works fine – contact cards, meetings, voice mail etc. But there is exclamation mark in low right corner of messenger.
Problem: if you ctrl+right click at SfB icon in systray and open configuration information, you can see that “EWS unavailable”.
Exchange web services works fine. Skype for business trace logs are clear, some warnings, but no errors at all. Then i decided to run fiddler. There was some troubles, but EWS were available and all sessions has been established successfully. I decripted EWS answers and found some strange garbage from the past – parts of conversations, nonexistent mailboxes etc:
So i archived all conversations away from exchange mailbox. Also launched search “missed conversations” across all mailbox, then moved this messages to archive also. Archive is dedicated .pst on local machine. Problem solved!
Explanation: this is old account and old mailbox, renamed on several ocasions. So, there was many conversations from email@example.com. Skype for business client could not decide what to do with that conversations in firstname.lastname@example.org mailbox and gave me error “EWS unavailable”.