the ports need to register to work, that's why the solution for this is the CME-AS-SRST.
a "pure" SRST assumes the communication has been lost and calls will be routed thru PSTN but at main site CUCM is up and running along with Unity and they're both communicating fine (ports are registered)
pure SRST won't register the unity ports, if you want this then you'll need to use CME-as-SRST
Actually the Unity server would be located on branch site as described in the cisco document
Integrating Cisco Unity with Cisco Unified CME-as-SRST
So Unity will not be communicating with CUCM when in SRST mode, as described in the document. So what would make the registration to a "pure" srst gateway impossible, while it would be on a "CME-as-SRST" ?
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