The full form of ISDN is Integrated Services Digital Network. It’s used on Technology ,Communication in Worldwide

ISDN or Integrated Services Digital Network is a circuit-switched telephone network system that transmits both data and voice over a digital line. You can also think of it as a set of communication standards to transmit data, voice, and signaling.

These digital lines could be copper lines. It was designed to move outdated landline technology to digital.

ISDN connections have a reputation for providing better speeds and higher quality than traditional connections. Faster speeds and better connections allow data transmissions to travel more reliably.

The modern upgrade to an ISDN would be a cloud phone system or SIP Trunking—they use the internet for phone service.