No, there's no way to force the incoming call to forward to voice mail right away once the call's been offered already (i.e. by the time they know who it is) at the phone.
Unity does offer "Call screening" features if the call is being transfered through the Unity auto attendant - Unity asks the caller to say their name and that name is played to the target subscriber who then has the option of taking the call or sending them to voice mail.
This requires, however, that the call come through the Unity auto attendant - there's no way to do this if the call comes into the phone directly without Unity knowing about it yet.
