“Telephony” refers to the technology, both hardware (equipment) and software (computer programs), that allows electronic communication over long distances. You might think that just means talking on the phone, but talking is only half the story.
Telephony is the science of taking an analog signal, converting it to electrical signals, transmitting it, then converting it back to an analog signal. In addition to voice, telephony also encompasses caller ID, fax, voice mail, and TTD (telecommunications devices for the deaf).
VoIP Telephony refers to all the services you expect associated with your phone, with some overlap from the services you can expect from your computer. With VoIP services, you can use computer features with your smartphone, and use phone features on your computer. For example, you can check your voice mail on your computer, send a virtual internet fax, send a text with your computer, or chat on your phone.