Telephony is the whole business of sending voices over a distance — the systems, the equipment, and the service, taken together as one idea. It is not a single phone and not a single call; it is the technology as a field. Engineers, phone companies and IT people use this word: 'mobile telephony', 'internet telephony', 'the telephony market'. An ordinary person almost never says it — they say 'phones' or 'phone service'. So the feeling is technical and slightly formal, the kind of word you meet in a company report, a news article about a telecom company, or a job title like 'telephony engineer'. You will very often see it with a word in front that names the kind: 'digital telephony', 'IP telephony' (calls carried over the internet instead of old phone wires). Note that it is uncountable — you do not say 'a telephony' or 'two telephonies'.