Simple with CRC checking. Y modem - Faster and more sophisticated than X modem, Y modem is particularly designed to handle batches of files. Z modem - Faster and more sophisticated than Z modem. CTI - Computer Telephone Integration Computer Telephone Integration referrs to the equipment on your premises. At the telco, this integration was done long ago.
Telephone switches are now just big computers.
The advantage of voice mail over an answering machine is that many individuals can have their own private mailboxes and not have to hear each other's messages.