telephony vs voicemail

telephony and voicemail both are nouns.

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

As nouns, voicemail is a hyponym of telephony; that is, voicemail is a word with a more specific, narrower meaning than telephony:

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

Other hyponyms of telephony include voice mail, call, phone call, telephone call, call forwarding, call waiting.

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