Can you please indicate what is the call flow? are all calls passing through CUC first then the end user? or are those DIDs?
If caller is passing through CUC then you can setup the main call handler to switch to the alternate greeting (containing the emergency prompt).
If you have a main number with an operator you may transfer this to a CUC call handler that does the same as above.
If these are DIDs, you may use a BAT pre-created file to do this remotely.
Any more detailed info is useful!