After reviewing messaging failover options I see two possibilities for a centralized deployment of CUC.

1) CUC with SRST - When the WAN becomes unavailable, the remote SRST router can be configured to route unanswered calls over the PSTN to the central CUC for messaging services.

2) SRSV - Remote site hardware (SRE or VM) can be deployed to provide messaging services in the event of a WAN outage.

My question is, why would any business incur the expense of SRSV hardware and licensing when the first option above is essentially available at zero cost (other that perhaps centralized site PRIs)?  Is there any significant functionality that SRSV provides over the SRST method described in option 1 above.

I don't get it.