This is the broad term used to refer to connecting computers and telephones.
Examples: retrieving your balance from your bank via phone, voicemail. Single line telephony is supported under Linux using vgetty, but more advanced multiline applications are not. At least not until one of the major board makers (Dialogic, Rhetorex, Natural Microsystems) either makes drivers, or provides specs.
One more thing that SCO and Windows NT do that Linux does not.