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 |


CTO @ Golden Five | CEO at LAExUG Foundation

Prabhat is 3 times Microsoft MVP Award winner. He is MBA in Information Technology and he is working as a CTO at Golden Five Consulting which is a Microsoft Gold Partner, MSP, T1CSP, and Education partner. He helps in designing, implementing, managing and supporting solutions for private messaging cloud, mergers, a collaboration between different messaging software and other migration & deployment projects for the following technologies Office 365, Azure, AWS, Exchange, SQL, ADFS, MFA, FIM, MIM and Directory services. He has worked for all big IT giants either as an employee or contractor where he has led the Global teams. He has started his career as Technical Consultant in Exchange 5.5 with Microsoft PSS and his exchange love never stopped & continued with 2000/2003/2007/2010/2013/2016/2019/O365. At one point in time, he was the only person to support EDS customers when Microsoft had closed all the supports for 5.5 and now for all old legacy Exchange versions.
He used to blog at, manages multiple LinkedIn and Facebook Groups. He also Owns MSExchnageGuru YouTube channel where he uploads all his records technical sessions. Don’t forget to check his PowerShell scripts which are making admins life easier. Prabhat can be reached at

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