My modem is an external USR 5686E model.

Since no Linux distribution includes a ling from /dev/modem to
/dev/ttyS0 or whatever channel the modem is installed you need to
create a symbolic link (as root) such as the following:
lrwxrwxrwx 1 root root 10 2008-11-09 08:26 /dev/modem -> /dev/ttyS0