Known as "calls", "call signs", "call letters", "station I.Ds.", and the FCC's designation - "Call Sign Assignments", what they are is NAMES identifying individual radio stations.
In the Homeland, formerly the U.S., radio station call signs in the present day consist of four alphabet letters beginning with "W" for stations east of the Mississippi River and "K" for those toward the west.
What does the Mississippi River have to do with it? That's not what I'm here to discuss.
Prior to an earlier year K and W was assigned without regard to location, which is why legacy stations KDKA Pitsburgh, Pennsylvania, and WEW St. Louis, Missouri, are on the wrong side of the river.
No matter what side of the river Part 15 stations are on, they are not assigned call letters by the FCC. But, there is an FCC Rule governing the naming of Part 15 unlicensed radio stations.
Part 73 - RADIO BROADCAST SERVICES
(1) Users of nonlicensed, low power devices operating under part 15 of this chapter may use whatever identification is currently desired, so long as propriety is observed and no confusion results with a station for which the FCC issues a license.
A question was raised by Rich on this website regarding our use of the calls KDX to identify our part 15 operation, based on the notion that we might be confused with a licensed station. My answer at that time was in two parts: 1.) To avoid confusion I adopted the practice of adding the number "1", saying "KDX1", when referencing my part 15 stations, but, 2.) since the FCC does not require part 15 operators to broadcast any identifying information, I don't.
Today I am considering a different answer to Rich's original question... I am considering the thought that using letters of a non-existent station, in other words, letters not currently assigned by the FCC to any station, and in fact "KDX" is not known to be assigned to any licensed station, it would be "reasonably proprietary" to use such call signs for a part 15 station.
This subject arises from my intention of attaching separate calls to each of my stations, rather than calling them all by the same (kdx) name.
Already I have established KHZ 1640 AM, the official radio station of the indoor antenna experiment.
1550 AM is now KEGO, EGO - the Station Where You Listen to Yourself.
One of my FM channels has become KLPH-FM, the station where I edit and produce The LPH (Low Power Hour).
None of these call letters show up at radio-locator.com and are presumed to be officially unassigned.
One call-set I hoped to use was KCAM, the name of my original part 15 station back in 1957, in which I appointed myself the AM translator station for local KCFM, a licensed station. But today there are two KCAMs, one AM the other FM, assigned to Glennallen, Arkansas.
But if Rich and others still oppose using letters that resemble FCC authorized stations, maybe the part 15 community should self-impose some kind of restriction against "K" and "W", and have our own letter, such as "O."
I sort of like that: "This is ODX - Oh 1550."