Hi,
There is a solution based on a modified Voicemail script.
https://wiki.innovaphone.com/index.php?title=Howto:Simple_caller_notification_Voicemail_XML_Script
Let the call route with a CFNR to this modified voicemail script.
Instead of a normal announcement, play an announcement that gives first a few times a ringback tone (with this, the caller doesn't notify that he is already in a voicemail system) and than a message that the called party is not available right now.
The modified voicemail scrip will however send a message to the called party email-address.
BR,
Karl