Did you know the difference between ISDN National and Custom configuration? I often get requests for legacy ISDN telephone sets and what people don't often realize there are two types of configurations. Even with the advent of IP, ISDN service is still widely utilized in government and municipal applications because of its flexibility, economy, quality, options, and inherent security. National configuration is often found with service offered through phone companies providing service through Northern and Siemans switches. The most common platform is Custom configuration through the AVAYA-Lucent-AT&T 5ESS switch. Before you order any ISDN service, phones, or parts for ISDNusage please be aware of the two types of configurations in the ISDN world.