Voicemail announces extension posing a security risk

When users call an extension the voicemail prompt gives that extension number. The extension number is the same as the room number. Running a student dorm, this allows someone to ask to be transferred to a person and that transfer provides the room number (which we do not give out). If an ex____ (fill in the blank) is trying to find where this person is, they now have the room number. Anyway to stop this in the standard prompts?

Forse the users to record their voicemails and make sure announc
e extensions is off in the directory.

keep in mind each extension has a name, unavailible, and busy greeting. all 3 need to be recorded or make sure routing always goes to unavailible greeting.