I have some suggestions if you are asking this for compliance reasons.

There is no reporting on the custom voicemail, but you can control what sort of voicemail message the users have by removing their “Voicemail Settings” permission. 

You can do a similar thing for forwarding - remove the call handling permission if you don’t want a user to be forwarding. 




I am not aware of there being a way to report on users who are forwarding, but you can try to narrow down the users you need to check by doing this trick:

Create a dummy call queue
Add all users as a call queue member.
you can then Expand the queue as demonstrated in pic below and view the status of each queue member. 
If a user appears Available - they definitely don’t have any forwarding at the moment.
If they appear Unavailable - they might have forwarding, or be on DND or they might be out of hours.