Exchange Online: Unable to Enable Unified Messaging


Old user joined the back, customer set all properties but failed to enable the Exchange Online unified Messaging for the message waiting indicator.

Error 1: We found the following error in the Office 365 Admin center:
Exchange: An unknown error has occurred. Refer to correlation ID: 5caf0222-dd65-427e-bdd0-ad977a4c6592

Error 2: We found the following error in enabling Unified Messaging:
Unified Messaging can’t save the PIN for mailbox ‘User email’ : 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. .

We found that user was hide from address list.


  • Go to on premises exchange admin center, find the recipient, uncheck the check box (Hide from address lists) to unhide the user from address lists.
  • Sync the AADConnect.
  • Once AADConnect update the user properties, we should be able to enable the Unified Messaging for the end user.

Prabhat Nigam
CEO @ LAEXUG Foundation | CTO @ Golden Five Consulting |


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