Traditional Phone Systems
People still use and buy traditional phone systems today using ISDN2e and ISDN30. There is nothing wrong with that approach where it is appropriate. Nothing at all.
But for most businesses and organisations, there are some really valid and compelling reasons to deploy more up-to-date solutions such as VoIP, Cloud, SIP, and Hosted Internet based telephony.
Frankly, ISDN is now well over 20 years old and is expensive by comparison to SIP or Hosted. Plus you have BT Openreach in the mix and no one should endure that if it can be avoided. Plus ISDN is end of sale in 2020 and being retired in 2025