I have an ISDN connection and would like a way of running the queue automatically when it is up.

The following shell commands test for the interface being up and then run the queue:

ifconfig ppp0 | fgrep UP >/dev/null
if [ $? -eq 0 ] ; then exim -q ; fi

You could put these commands into a script which runs them at regular intervals. You might want to use -qq instead of -q. With Linux, the script /etc/ppp/ip-up is run after a ISDN connection or a more general PPP connection has been established. If you are using Linux, you could put the call to Exim in that script.