ISDN (Integrated Services Digital Network)

Steve says:

An international standard for high-speed, low-noise digital phone lines that can be used for voice or data. Two forms are available: A Basic Rate (ISDN2) circuit provides 2 x 64K lines. A Primary Rate (ISDN30) circuit provides a minimum of 8 and up to 30 x 64 K lines.

Doris says:

You won’t find these in your home but businesses all have them connected to their phone systems. And all this time, Steven had me believe it stood for 'In Severe Domestic Need'.