If you need this you can use a waiting queue to play audio and disconnect call.

The user will redirect the call to the WQ instead of the voicemail.

Within the 13r3 you have now the possibility to configure CF directly in the Phone/softphone app via the menu diversion

if the user want to do its own announcement you can use this :

https://wiki.innovaphone.com/index.php?title=Howto:V13_Record_an_announcement_by_phone

you can also upload since the 13r2 a custom voicemail script into the voicemail app, I imagine to have only the necessary part of the voicemail script to play the announcement only.