A voicemail only extension is a handy extension where you want calls, depending on their setup (either through Auto Attendants, Hunt Groups or Call Queues), to go directly to voicemail rather then to a user.
Setting up a voicemail only extension is the same as any extension - the only difference is that you do not want anybody to register or login using that extension.
All you need to do is toggle the Enable Voicemail to ON. You can then upload a greeting ton inform callers to leave a voicemail.
For added effect, you can enter an email address under the Email field where the voicemail, together with the transcription will be sent whenever someone does leave a voicemail.