Transmitters Requiring No Call Signs
No call signs are issued to transmitters of long-range navigation systems (Decca, Alpha, Omega), or transmitters on frequencies below 10 kHz, because frequencies below 10 kHz are not subject to international regulations. In addition, in some countries lawful unlicensed low-power personal and broadcast radio signals (Citizen's Band, Part 15 or ISM bands) are permitted; an international call sign is not issued to such stations due to their unlicensed nature. On some personal radio services, such as Citizen's Band it is considered a matter of etiquette to create one's own call sign, which is called a handle (or trail name). Some wireless networking protocols also allow an SSID to be set as an identifier, but with no guarantee that this label will remain unique.
International regulations no longer require a call sign for broadcast stations; however, they are still required for broadcasters in many countries, including the United States. Mobile phone services do not use call signs on-air for obvious reasons ; however, the U.S. still assigns a call sign to each mobile-phone spectrum license.
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