The above reply is correct, there is no present feature for this (which is a real shame). To date, if the SRST site is down for any length of time, we reconfigure their calls manually to go back out across the PSTN to reach them. How well this works really depends on the ability of the remote site to receive calls in terms of DDI. If thats not possible, the current VM alternative is the best option.
We've been asking Cisco for this feature since the early CCM 3.0 days, but nothing so far. It shouldn't be too hard to build in some logic to identify a whole location/device pool suddenely becoming unregistered and take an alternative PSTN path. They could build in some kind of retry (just incase its a genuine site down), to use VM, maybe after 3 attempts. I've come across the same issue with WAN based clustering, which in fact is even more difficult to configure a workaround because you can't make changes to the islolated subscriber. SRST actually works better in those scenario's (with the exception of feature availability). I really wish Cisco would sort this out.
Recently I've been testing a way of using AAR to force the central publisher to redirect calls in the event of WAN failure by manually changing the location bandwidth down to 1 when a failure occurs to force an AAR condition. I haven't yet got it working, but I need to spend a little more time on it.
