I remember something in an old Phrack or another hacker zine where someone was recounting a rumor about this mechanism that he heard from someone who worked in a telco (viz., that there were special government phone numbers that were treated differently by the telephone system). It's interesting to see the progression from underground rumor to Wikipedia article.
There used to be a designation for landlines where they would be unlisted and have priority when there was congested outgoing lines or a shutdown.
I had a relative who was a public safety official. It was pretty wacky, they had an old school western electric leased phone, this magic line and other weird features like custom short codes that would hit "internal" extensions in the office.
