“Tavajoh! Tavajoh! Tavajoh!” a man’s voice announces, before going on to narrate a string of numbers in no apparent order, slowly and rhythmically. After nearly two hours, the calls of “Attention!” in Persian stop, only to resume again hours later.

The broadcast has been playing twice a day on a shortwave frequency since the start of the US-Israel attack on Iran on February 28.

According to Priyom, an organization which tracks and analyses global military and intelligence use of shortwave radio, using established radio-location techniques, the broadcast was first heard as the US bombing of Iran began. It has since played on the 7910 kHz shortwave frequency like clockwork—at 02.00 UTC and again at 18.00 UTC.

Over the weekend, Priyom said it had identified the likely origin of the broadcast. Using multilateration and triangulation techniques, the group traced the signal to a shortwave transmission facility inside a US military base in Böblingen, southwest of Stuttgart, Germany.

The site lies within a restricted training area between Panzer Kaserne and Patch Barracks, with technical operations possibly linked to the US army’s 52nd Strategic Signal Battalion, headquartered nearby.

That identification narrows the field, but it does not reveal who is behind the transmissions or who they are meant for.

The two-hour-long transmission is divided into five to six segments, each lasting up to 20 minutes. Each opens with “Tavajoh!” before shifting into a string of numbers in Persian, sometimes punctuated with an English word or two. Five days into the broadcast, radio jammers were heard attempting to block the frequency. The following day, the transmission shifted to a different frequency—7842 kHz.

Radio communication experts believe the broadcast is likely part of a Cold War–era system known as number stations.

The Return of the Numbers

Number stations are shortwave radio broadcasts that play strings of numbers or codes that sound random—like the one now heard in Iran. “It is an encrypted radio message used by foreign intelligence services, often as part of a complex operation by intelligence agencies and militaries,” says Maris Goldmanis, a Latvian historian and avid numbers stations researcher.

Number stations are most commonly associated with espionage. “For intelligence agencies, it is important to communicate with their spies to gather intelligence,” says John Sipher, a former US intelligence officer who served 28 years in the CIA’s National Clandestine Service. “This is not always possible in person due to political constraints or conflict. This is where number stations come in.”

While the use of number stations can be traced back to the First World War, they gained prominence during the US-Soviet Cold War. As espionage grew more sophisticated, governments used automated voice transmissions of coded numbers to communicate with agents, Goldmanis says. Citing declassified KGB and CIA documents, he adds that number stations were widely used during this period, often as Morse code transmissions and, in many cases, as two-way communications, with agents reporting back using their own shortwave transmitters.

“Nowadays, you have various satellite and encrypted communications technologies,” Sipher says. “But during the Cold War and even before that, governments had to find ways to do this without being noticed, and broadcasting coded messages was one way to communicate with your assets discreetly.”

The apparent randomness of the numbers means they can be understood only with a codebook, Sipher adds. “Nobody can make heads or tails of it or understand what it says unless you have the codebook that can give you hints to decrypt the code,” he says, noting that such systems must be set up and coordinated in advance.

A Signal Without a Sender

While the likely origin of the signal may now be clearer, its purpose and intended recipient remain unknown.

Because the broadcasts are encrypted and designed to be covert, those details may remain unclear for years, Goldmanis says. The structured nature of the transmission—its fixed schedule and consistent use of frequencies—further suggests it is part of a planned operation.