Thank you for the answer. Yes, I'm root on the machine where I run the script. This is the permissions of the special character device /dev/usb/lp0:

crw-rw---- 1 root lp 180, 0 Oct  3 13:14 /dev/usb/lp0
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and yes, I can write into the device

pbx:/usr/local/share/router# echo "test" > /dev/usb/lp0 
pbx:/usr/local/share/router#
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I'm using Debian Lenny. Thank you again.