Customer is in the Exchange Hybrid Configuration with Office 365 and Exchange 2016 CU 11.
While enabling the UM mailbox, customer got the following error:
Unified Messaging can’t save the PIN for mailbox “”: There was an error accessing the mailbox using EWS. The requested operation failed with ResponseCode “ErrorInternalServerError”. Details : An internal server error occurred. The operation failed., Access to Active Directory failed. .
This mailbox has hide from address book checked. So, Exchange online could not enable UM Mailbox for this user.
Logged in to the Exchange 2016 ECP, unchecked the checkbox “hide from address lists” to show the user in the address book.
Then replicated the user properties to office 365 Exchange online using AADConnect.
Now Enabled UMMailbox.
Once UMMailbox was enabled, checked the checkbox hide from address lists and replicated user properties to the office 365 Exchange online using AADConnect.