Call Appearances is a feature used on many multi line telephone systems (or MLTS) to allow a single telephone to handle multiple calls at once without having to be bombarded with an annoying call waiting tone.
Central Exchange (or Centrex) is a service provided by many telephone carriers to deliver PBX or Key-like services via the telephone company (or Telco.) Little to no hardware is required at the customer end, unless its special adjuncts for digital telephones for power, etc. Regardless of the hardware, the “brain” lives out in the “cloud” so to speak at the central office. If it’s a government, or a large corporation, rarely are these actually installed on site.
There’s a difference between three way calling, loudspeaker calls and the ones you hear quarterly by the publicly held companies.
A teletypewriter is a special device that was typically known as the Telecommunications Device for the Deaf or TDD. These devices act as typewriters that can carry special signals to provide text based communications over voice telephone lines. This was the beginning of using data over the telephone lines.
You think it’s just a modified PC with an application that runs something similar to an answering machine? You may be right, but did you realize that for many years it had a oh and ten record in terms of success? For almost twenty years, voice mail was dead on arrival. History can be seen on the link.