I don't have an Elk but I guess you could define the telephone reporting format as 5 (voice message) and enter the number to dial as 33* (you need to have the phone interface of the Elk plugged into an extension port on the phone system and program that port to work with single line phones).
Then in a rule you have some trigger and for your then action it would be THEN DIAL Central Station [33*] AND ANNOUNCE Miscellaneous 1[VM 239]
(substitute whatever voice message you want to use in place of Miscellaneous 1. Just make sure if you use a Miscellaneous voice message that you go define it with some words.)
Anyway this is how it looks like it should work but I don't have an Elk so I could be wrong.