telephony vs voice mail

telephony and voice mail both are nouns.

Word	Noun	Adjective	Verb	Adverb
telephony	Yes	No	No	No
voice mail	Yes	No	No	No

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

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

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

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