voicemail vs telephony

voicemail and telephony both are nouns.

Word	Noun	Adjective	Verb	Adverb
voicemail	Yes	No	No	No
telephony	Yes	No	No	No

As nouns, telephony is a hypernym of voicemail; that is, telephony is a word with a broader meaning than voicemail:

voicemail: a computerized system for answering and routing telephone calls; telephone messages can be recorded and stored and relayed
telephony: transmitting speech at a distance

Other hypernyms of voicemail include telephone.

voicemail (noun)	telephony (noun)
a computerized system for answering and routing telephone calls; telephone messages can be recorded and stored and relayed	transmitting speech at a distance