Dude, you have the strangest requests!!
If you don't even want the LED to be visible, you have to NOT assign the line to the phone
the first thing you do is set the number of lines on the system to 1, which will automatically assign Line 1 to ALL extensions with IMMEDIATE ring and be the FIRST line selected when going off hook.
Then you ASSIGN Line 2 to Ext. 17, and it appears on the 2nd button, after Line 1. It will have IMMEDIATE ring and be the 2nd line that is selected when lifting the handset (after Line 1)
Then ASSIGN Line 3 to Ext. 18, it will appear on the 2nd button, after Line 1, with IMMEDIATE ring and be the 2nd line that Ext. 18 selects when lifting the handset (after Line 1)
From Ext. 10:
Feature 00, Left Intercom Twice to enter System Programming
#104-01 (This sets the number of system-wide lines to Line 1, so EVERY phone gets Line 1 on button 1 with IMMEDIATE Ring and Auto Line Select picks Line 1 first)
#301-17-02-1 (This assigns Line 2 to Ext. 17)
#301-18-03-1 (This assigns Line 3 to Ext. 18)
Feature 00 to exit