Hi, Here is a brief explanation of SPIDs taken from an email I saved long long ago. Hope it helps... -=fade=- There's not much to it. Simply put, SPIDs identify to the phone company what types of services and features are supported for a given device. SPIDs are optional in the ISDN standard, but usually required in North America. As I said above, SPIDs are usually 12 digit numbers consisting of the area code, 7 digit number and some trailing 0s and/or 1s. For example: 408 555 1212 00). The DN (Direct Number) is simply the "normal" 7 digit phone number. For example: 555 1212. There are three switch types the LS-ISDN and NI2B Dual-BRI card support: AT&T 5ESS, Northern Telecom DMS100 and National ISDN-1 (NI1). If a ISDN service provider is using an AT&T 5ESS with a software revision lower than 5E8, then most likely the 5E can not support NI1. In this case, SPIDs are not required. However, if they have the 5E set-up for NI1, they will tell you to set up your equipment for NI1 and they will provide SPIDs and DNs. Same for the DMS-100...expect that SPIDs and DNs are usually always required for the DMS-100, regardless if the switch is setup for NI1 or DMS-100. A good ISDN home page on the Web is: http://www.cis.ohio-state.edu/~fine/ISDN/ Also, http://alumni.caltech.edu/~dank/isdn/ Hope this is helpful. Jim Tremolini Senior Systems Engineer Telebit Corporation