voice mail vs telephony

voice mail and telephony both are nouns.

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

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

voice mail: 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 voice mail include telephone.

voice mail (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