ISDN

Integrated Services Digital Network (ISDN) line uses a standard copper telephone line to transmit voice data and connect to the public telephone network. It was first introduced in 1986 and along with the
PSTN will be phased out by BT in 2025, though still remains surprisingly common. ISDN was considered cutting edge because it transmitted call data digitally instead of using analogue signals, which greatly enhanced call quality.
An ISDN line enters a premise and connects directly into a box that separates the data into channels, 1 channel = 1 call, the box then connects to a PBX using an Ethernet cable, which then connects to the ISDN telephones. The maximum number of channels that an ISDN box can support is 30, if the client wants more channels/phone lines then they can add another box or purchase another ISDN line. ISDN lines can take a long time to provision, only work with specific hand sets and the phone numbers are geographically linked to the line itself, so cannot be easily moved or used by another device.