Turn off both in the settings and dial *71<talkatone#> and press Call, this will use VZW's conditional call forwarding feature to send calls to on a busy, no answer, reject, or no response (no network/powered off) condition to the target number. Just do a *73 and press Call to end Call Forwarding and return to a default state (VZW Voicemail). Works the same for Google Voice, actually this is the way GV recommends setting up GV VM on Verizon.