This can vary depending on how your system is set up. Intercept handling is one way to do it. See what Intercept number is set for the trunks in Trunk Attributes, then go into Intercept Handling and see what's set for either "Directory Number Out of Service" or "Unassigned Directory Number" for that Intercept assignment number, that will tell you what DN the call is being sent to, which should translate to either a RAD or voice mailbox that's playing your message.