And if you're doing THAT then Twiga's free "Remote Disconnection Utility" from
http://www.twiga.ltd.uk/ can be helpful.
"Do you share a single Internet connection among PCs? Many do, and one of the biggest problems is keeping track of when the shared dialin connection is up and down. Believe it or not, some people on this planet still pay for Internet access by the minute, so something like Remote Disconnection Utility is just the ticket. It will allow you to drop the modem connection from a remote PC, which should keep those connection costs down considerably."
[It can also do a lot else, such as lock a connection, permit or deny connection/disconnection/reconnection requests and more. You can download a modified module from Twiga which recognises ISDN connections too - I think this replaces one of the base modules installed by rdusetup.exe]