ISDN is the technology that brought digital functionality to the traditional phone network, otherwise known as the Public-Switched Telephone Network (PSTN). In particular, ISDN enables the simultaneous transmission of different types of data, such as voice and video, over the PSTN.
In the near future, ISDN will be decommissioned across Europe in favour of voice-over-IP (VoIP) technology, which routes phone calls across the internet rather than the PSTN. In the UK, BT has already announced that ISDN will be decommissioned in the next few years, while the date set for decommissioning in Germany is 2018. VoIP technology is far more efficient and cost-effective than ISDN, which is one of the main reasons it is being decommissioned.