Note:You can drill down this process, if you us OCSLogger and Snooper catching the OPTION and 200 OK communication directly on the SBA.
Voice Mail DepositIf 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).
Voice Mail Retrieval
New-CsVoicemailReroutingConfiguration -Identity Site:Ingolstadt
Set-CsVoicemailReroutingConfiguration -Identity Site:Ingolstadt -SubscriberAccessNumber “+49899090123″ -AutoAttendantNumber “+49899090456″ -Enabled $true
Thanks to Justin Morris for his detailed explanation