What is the difference between Exchange Online Unified Messaging and Exchange 2010 on prem Unified Messaging?

This was a question asked by a large university in Ohio moving towards Office 365.


Diagram showing non-SIP based PBX connecting to Exchange Online Unified Messaging


The good news is Exchange Online UM is almost the same in functionality to the Exchange 2010 on premises UM offering.  It offers the following capabilities:

  • Call answering
  • Outlook Voice Access
  • Automated attendant
  • Message Waiting Indicator for on premises phones
  • Text transcription of all voicemail
  • Play on phone


What are the differences between Exchange 2010 UM on premises vs. Exchange Online UM offerings? 

The major feature differences in UM capabilities are around speech recognition. Three areas that are different:


1) Autoattendants with Exchange Online UM are DTMF only. Speech works with on prem Exchange UM only

2) Directory look-up with Outlook Voice Access is also DTMF only. Speech recognition works with on prem Exchange UM only

3) Not all UM powershell cmdlets not available such as UMserver cmdlets.


Other functionality differences:

  • Management interfaces If your UM deployment is cloud-based only, you use the Web-based Exchange Control Panel to manage UM. In a coexistence scenario, where you have both on-premises and cloud-based UM users, you use the Exchange Management Console.
  • UM server requirements If your UM deployment is cloud-based only, you don't need to set up and maintain UM servers on-premises. You use the UM service in the cloud instead. This means you don't have to associate your UM dial plans with a UM server, and many server-side configuration options, such as language pack management and server logs, aren't necessary. If you are setting up your organization for UM coexistence, you have to set up and maintain UM servers for your on-premises UM deployment.
  • VoIP security options In a cloud-based UM deployment, communication between your Private Branch eXchange (PBX) and the UM servers occurs over the Internet, so the VoIP security setting is automatically set to Secured. This enforces Transport Layer Security (TLS) and Secure Real-time Transport Protocol (SRTP) when VoIP traffic is routed between your on-premises session border controller (SBC) and the UM service in the cloud. In an on-premises UM deployment, you can choose the security setting.
  • UM IP gateway configuration When you configure your UM IP gateway in the cloud, point the gateway to your on-premises SBC, instead of directly to the IP PBX. For more information, see Create a New UM IP Gateway. In an on-premises UM deployment, you point your UM IP gateway directly to your IP PBX.
  • Forwarding address For an on-premises deployment of UM, you point your UM IP gateway directly to your UM server. In a cloud-based UM deployment, the UM server addresses aren't static, so you have to configure your SBC to point to a forwarding address for the UM service in the cloud. For more information, see Forward Calls from Your Physical Telephone Device to UM.
  • Licensing For Microsoft Office 365 for education, you must have a subscription license that includes UM for the mailboxes that you UM-enable. You manage licensing in the Microsoft Office 365 portal. For more information, see Assign a License to New Mailboxes in Office 365.



Can I have a mailbox on prem with UM in the cloud or Exchange Online mailbox with UM on prem?

No, the rule to follow is wherever your mailbox resides is where your UM server must reside. You cannot have a hybrid mailbox and UM scenario.  You can however have some on prem mailboxes with on prem UM combined in hybrid fashion with Exchange Online mailboxes with Exchange Online UM

What additional hardware do I need to connect my on prem PBX to Exchange Online UM?

Your hardware requirements will vary depending on what you are using for call control:



SIP gateway – an appliance to translate TDM to SIP to talk to Exchange Online UM.  See tested SIP gateways with Exchange UM here.

Session Border Controller – an appliance for secure voicemail with Exchange Online UM



No SIP gateway needed provided your SIP PBX is listed here.

Session Border Controller – an appliance for secure voicemail with Exchange Online UM


Lync Server 2010 enterprise voice:

No additional hardware required other than Lync Edge server if not already present.

Can I use any SBC with Exchange Online UM?

We strongly recommend you leverage the tested SBCs that are known to work with our SBCs and our Exchange Online UM.  You can find the latest list of tested SBCs here.


Can I connect my Lync Enterprise voice deployment to Exchange Online UM?

Yes, it works and you can eliminate the need for an on prem SBC.   Here is a diagram:


Diagram of Lync voice on prem connecting to Exchange Online UM


For more information on deploying Exchange Online UM with Lync voice see here.


Where do I find the guide to setup Exchange Online UM?

Great information here.

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