The following piece was written by the late Havana Moon. It gives you an idea of what Numbers Stations are. Although it has been written a long time ago, it is still relevant.
"Spy Numbers Transmissions" are a shortwave oddity which have been around for the last 40+ years. They are commonly heard in English, German, Spanish and the Slavic languages and take the form of four and five digit groups of numbers which are preceded by a three digit "identifier" and a "group count" which corresponds to the number of number groups transmitted in the crypt. They are generally broadcast by a mechanical-sounding YL, although Morse code (CW) "cut number" transmissions are also frequently reported, as are phonetic alphabet transmissions. Several distinctly different formats have been noted. The "who" and "why" aspects of these transmissions are, for the most part, unknown. Their mysterious nature has resulted in their common characterization as 'spy' transmissions. The spy theory has been enhanced over the years by the FCC's inconsistent position in response to numerous inquiries by the Shortwave Listening community. If you've never heard a numbers transmission, tune your radio to one of the frequencies on the log-list. You'll almost certainly hear a 'classic' numbers transmission which will run from 15-30 minutes. In regards the "where" - a number of sites have been identified in recent years, notably in Warrenton and Remington, VA and Miami, FL. In addition, with the aid of sophisticated RDF (radio direction finding) equipment and the invaluable assistance of highly authoritative and professional resources, several new transmission sites have been posi- tively identified over the past years, including sites in Nicaragua, Honduras, El Salvador, Cuba and Cozumel. One particularly dangerous station has been interfering with air to ground traffic on 6577 kHz, a frequency allocated to international aeronautical communications in the busy Caribbean sector. On at least one monitored transmission, the air traffic controller at ARINC moved the pilot to an alternate frequency as the numbers transmission was totally blocking the frequency from effective use. An identified Cuban site (Guineo) is believed to be a major transmitter site used by DGI (Cuban Intelligence). There has been some suggestion that the numbers are a form of 'one-time- pad' - a crude but unbreakable form of encryption (unless you've got the key, that is) - if that's the case, it's very curious to note that the same five-digit groups are often seen to repeat over and over again in the very same crypt - and that the same transmissions are seen to repeat from week to week, and from month to month - so often, in fact, that tape breaks are sometimes noted. When spliced back together, the sloppy handling sometimes results in truncated 5-digit groups - the end result being amixture of 4 and 5 digit groups in the very same transmission! These facts would tend to point one away from the one-time-pad concept and support a couple of other theories - suggesting that the numbers are NOT a cipher, but rather a code unto themselves, and that much of this traffic is 'dummy' in nature - broadcast simply to keep a frequency open over a long period of time. In addition, most five-digit Spanish numbers transmissions are very badly over-modulated, resulting in numerous spurs up and down frequency. When broadcast under such conditions, the numbers (6) seis and (7) siete are almost indistinguishable, making it impossible to copy a crypt without numerous errors. It is worth noting that the four-digit Spanish and English transmissions do not reflect this same technical ineptitude.
In 1987 Havana Moon wrote “Uno, Dos, Cuatro” a book about numbers stations. It has been out of print for years but a fellow dxer scanned it and sent it to me for the N&O website.
“Uno, Dos, Cuatro”
(PDF opens in a new page)