ISDN

 Integrated Services Digital Network

Allows simultaneous digital transmission of voice, video, data, other network

services over public switched telephone network

Circuit-switched telephone network system which also provides access to packet switched networks

Results in potentially better voice quality than an analogue phone can provide

One from:

ISDN video-conferencing systems still in use as high-definition is achievable using a Primary Rate Interface (PRI)

Is very cost prohibitive and can achieve the same results with SDSL for a lot

less.