Lync Voice Mail Survivability (SA) with Exchange Unified Messaging

The SBA is aware of the centrally configured Exchange Unified Messaging component. Additional it is important to understand the process of Voice Mail Survivability. While the VoicemailReroutingConfiguration in Lync provides the SBA with an alternative path, rerouted via PSTN to the central Exchange UM site.

The SBA is continuously check the SIP availability of the Exchange UM component, using the SIP OPTION based health checking. (Indeed, both SBA and non SBA Lync registrars act in the same way for all components, e.g. Exchange UM and other Gateway-SBC). Once the SIP OPTION command is sent, the sender expects an SIP 200 OK answer. Lync tries this health check several time even after it do NOT receive a 200 OK. As result of not receiving, the registrar will mark this target as DOWN and stop sending calls.

Note:You can drill down this process, if you us OCSLogger and Snooper catching the OPTION and 200 OK communication directly on the SBA.

Back to the Exchange UM, if Exchange is not responding, the SBA marks the UM SIP path as down and activated the configured PSTN reroute.


If a user SBA homed user is remotely connected to central site (his backup registrar), incoming PSTN call to this user will also be rerouted to Exchange UM VoiceMail.

A PSTN cannot be rerouted this this (backup) registered user!

Voice Mail Deposit

If a WAN outage occurs, the SBA has detected that UM connectivity is down, all unanswered or ignored call for effected user homed on this SBA will be routed over the PSTN to the assigned UM Auto Attendant (AA).

There is one change in user experience: Once the UM pickup this call, the AA will ask the calling party (either via speech or extension number) and then a voice mail can be recorded. This behavior is different to the native user mailbox response.

Voice Mail Retrieval

During a WAN outage, there are other service truly effected too, e.g. Exchange Server connection. But Exchange Subscriber Access is still available via PSTN calls. Therefor an effected user is NOT able to use his Lync Client for Voice Mail Preview, instead, the normal “Call Voice Mail” will place a PSTN call to the Exchange Server Subscriber Access Number (SA).

As long the user is assigned in AD and Lync with an E.164 phone number, Exchange is able to do a Reverse Number Lookup and can identify the users work number. Exchange then answer directly instead of asking for the user’s mailbox PIN.


Voicemail rerouting configuration must be established for all Branch Site equipped with a SBA

New-CsVoicemailReroutingConfiguration -Identity Site:Ingolstadt

For each Voicemail Rerouting, the Exchange Auto Attendant (AA) and the Subscriber Access Number (SA) has to be published.

Set-CsVoicemailReroutingConfiguration -Identity Site:Ingolstadt -SubscriberAccessNumber “+49899090123″ -AutoAttendantNumber “+49899090456″ -Enabled $true

Thanks to Justin Morris for his detailed explanation


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