dial up to network

There are a couple of ways of sharing an ISDN line. The simplist way would be to replace your current ISDN modem with an ISDN router. You can go to http://tinyurl.com/6ysw for a list of various manufacturers and models.

The low-end manufacturers like D-link and Linksys have moved away from ISDN to xDSL. However, you can probably pick up a used unit off eBay for less than $100.

You can also configure your current PC to share the line or act as a router. The particular OS that you will be running will determine the appropriate configuration.

Accessing your corporate site is a seperate issue from sharing an ISDN line. Policy and configuration issues at the main site will determine if you can share that, and if so what special configuration issues need to be addressed.

There is also a third alternative, much similar to the second already suggested method, but often cheaper - for small ( up to 10 PC ) networks. Beyond that number, the savings in costs are not worth it as the network will grind to a halt.

If the hardware replacement of the ISDN router proves expensive, you can install PROXY software on the machine thats connecting the ISDN line to the network. Then with static routing on the network ( DHCP will work in special circumstances if carefully monitored ) the other machines can access the remote resources through the services enabled on the PROXY software package e.g enabling SMTP on port 25, enabling HTTP on port 80 etcetc..... and so on .

Good luck and have fun, dont forget the fun part.
J.