An email will be sent if a voicemail is left.
If the call is simply missed (unanswered) and the caller doesn't leave a voicemail message then this is a missed call and no voicemail message is sent.
If they hang up the call before the voicemail message is played then this won't record it as a voicemail - instead it is recorded as a 'missed call' and hence no voicemail message is sent by email.