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:
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:
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:
Non IP PBX:
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.
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.