efax-gtk cannot do that for you. You should use udev to set up the permissions for your plugable devices. You can either lodge a support request with your distribution (it is a bug in your distribution not to set up permissions correctly), or write a udev rule yourself. This will tell you how to write your own rules: http://www.reactivated.net/writing_udev_rules.html
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
where in Preferences Efax I must put the command: sudo chmod 666 / dev / ttyACM0, because the automatically opening boot Efax port USB modem ???
efax-gtk cannot do that for you. You should use udev to set up the permissions for your plugable devices. You can either lodge a support request with your distribution (it is a bug in your distribution not to set up permissions correctly), or write a udev rule yourself. This will tell you how to write your own rules: http://www.reactivated.net/writing_udev_rules.html