Settings → System → Voicemails is really a small recordings library: each entry is a pre-recorded voicemail message, showing its length, who added it, the date, and how many times it's been used. Use + New recording to add another message. Whichever recording is selected in a rep's Live dialer session settings (see Live Dialer & Parallel Dialing → Configuring Your Dialing Session) is what gets dropped automatically when Vokkl detects a call went to voicemail.