olemus
(a) kõnepost: kõnesõnumeid edastav e-posti analoog
(b) kõnekiri: kõnepostisaadetis
Wiktionary:
1. (uncountable) a computerized interactive system for storing, processing and reproducing verbal messages left through a conventional telephone network
2. (countable, by extension) an individual message on such a system
Synonym: voice message

ülevaateid
https://www.3cx.com/pbx/voicemail-system/
https://en.wikipedia.org/wiki/Voicemail
https://en.wikipedia.org/wiki/Visual_voicemail
https://www.voicemailoffice.com/news/the-history-of-voicemail/

turvalisus
https://www.whitlockco.com/security-risks-voicemail-system/
https://www.alliedtelecom.net/5-precautions-to-prevent-voicemail-system-hacking/
https://www.gsma.com/newsroom/wp-content/uploads//SG.20-v2.0.pdf