I’ve just been setting up the Gnome modemlights applet on a Debian box.
To make it work, you need to set the preferences as follows:
Device: ppp0
Lock file: /var/run/ppp0.pid
Disconnection command: poff -a
and make sure that /dev/modem exists and is a link to your modem device.
Gnome ModemLights applet in Debian
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