As far as troubleshooting ISDN LAN Modems/routers, ask the following questions (the answers will give you insight on how to troubleshoot the call)

What LAN Modem/router is in use
What is at fault (all internet access, or something in particular)
How many workstations are affected
How long had the problem been occuring

As far as ISDN line information is concerned, switch type and directory/SPID numbers are provided for by the local telephone company, not the ISP. If the user does not know them, the user needs to get this information from the local telephone company in order to get an initial LAN Modem/router configuration working properly (but usually SPID numbers are written near the ISDN phone jack).

If the user has not done so, have the user turn off the LAN Modem/router, wait a minute, then turn on the power to the router (also known as recycling/restarting the device). Then test the internet conenction (web/mail).

If the entire network has no internet conenction, go into the configuration of the device (either via telnet/web browser/configuration application, depending on the product). If the configuration does not appear, recycle the device, then ping the device. If you get request timeout, there is a problem communicating to the device. If you get replies back, try to go back to the configuration
of the device.

If the user does not know how to get into the configuration of the device, find the default gateway (windows 95/98 - found via winipcfg and select appropriate ethernet adapter; on NT/2000, go to command line, type in ipconfig, and look for an ip address under an ethernet adapter). Usually, the default gateway will refer to the router. Ping the default gateway via a command line prompt; if a request timeout occurs, recycle the device.

For the user who has a Macintosh computer, and the user does not know how to get into the configuration of the device, go into the tcp/ip control panel and find out the router address, then attempt to get into the configuration of the device (either via telnet/web browser/configure application, depending on the product). If the configuration does not appear, recycle the device and try again.

Before changing access numbers, note what format the access number is in (7, 10, 11 digits). ISDN phone number format will vary from telco to telco. For example, with PacBell ISDN (California), a call placed as seven digits will access the 56k circuit (112k max), while eleven digits will access the 64k circuit