[asterisk-users] Per user voicemail greeting

[listuser at spamomania.co.uk]

I'm struggle to answer a simple question. One user at extension 4000
wants a custom .gsm file to play for their mailbox. I can't figure where
to put it/what to set in voicemail.conf to achieve this:

voicemail.conf
4000 => 4000,system,voicemail at ....net

Relevant extensions.conf line:
exten => 2,n,VoiceMail(4000 at voicemail)

It all works fine, playing the system VM greating, but I would like to
use the custom .gsm for this user only. Can anyone help?