Voicemail For Call Waiting

My current setup has one Google Voice inbound line that is set to ring extension 701 when calls come in. This works fine, and if the call is not answered, it goes direct to the “Can’t take your call right now…” message for my voicemail. However, if I am on the phone and a call comes in and I don’t answer it via call waiting, the caller hears the system “The user at extension 701 is not available…” (I’ve never actually heard it, so I am just giving you the general idea as reported by callers then it goes to voicemail. How can I skip this system message and have the call simply go to my “Can’t take your call right now…” message?