https://www.mattblaze.org/blog/neinnines/

Compromised reveals that the FBI found that during at least some of the time the illegals were under investigation, the Russian numbers intended for them were sent not by a transmitter in Russia (which might have difficulty being reliably received in the US), but relayed by the Cuban shortwave numbers station. This is perhaps a bit surprising, since the period in question (2000-2010) was well after the Soviet Union, the historic protector of Cuba's government, had ceased to exist.

The Cuban numbers station is somewhat legendary. It is a powerful station, operated by Cuba's intelligence directorate but co-located with Radio Habana's transmitters near Bauta, Cuba, and is easily received with even very modest equipment throughout the US. While its numbers transmissions have taken a variety of forms over the years, during the early 2000's it operated around the clock, transmitting in both voice and morse code. The station was (and remains) so powerful and widely heard that radio hobbyists quickly derived its hourly schedule. During this period, each scheduled hourly transmission consisted of a preamble followed by three messages, each made up entirely of a series of five digit groups (with by a brief period of silence separating the three messages). The three hourly messages would take a total of about 45 minutes, in either voice or morse code depending on the scheduled time and frequency. Every hour, the same thing, predictably right on schedule (with fill traffic presumably substituted for the slots during which there was no actual message).

If you want to hear what this sounded like, here's a recording I made on October 4, 2008 of one of the hourly voice transmissions, as received (static and all) in my Philadelphia apartment: www.mattblaze.org/private/17435khz-200810041700.mp3. The transmission follows the standard Cuban numbers format of the time, starting with an "Atenćion" preamble listing three five-digit identifiers for the three messages that follow, and ending with "Final, Final". In this recording, the first of the three messages (64202) starts at 3:00, the second (65852) at 16:00, and the third (86321) at 29:00, with the "Final" signoff at the end. The transmissions are, to my cryptographic ear at least, both profoundly dull and yet also eerily riveting.