ISDN is being phased out in favour of modern Voice over IP solutions, with signficiantly lower costs and better features it's easy to see why.
When your telephone system is running from the cloud it means you don't need to run and maintain your own physical PBX to handle calls, your phones connect straight to our network through the internet.
Reliablity is improved too, no longer are calls routed through one cable but can be easily re-routed through a backup internet connection or to another location.