We have a client who has a piece of software that creates a data-driven document and sends it via a fax. Yesterday, their phone company upgraded the telephone hardware. From that point on, their server stopped sending faxes. The server connects to the outside world via a direct ISDN link from the server to the ISDN line, or via an old robotics modem that connects to ISDN via a traditional phone socket.
The problem of not being able to send a fax was caused by unplugging the ISDN cable from the server without first shutting down the server. A simple server reboot fixed the problem.