Since 2155 where relay stations placed by the USS Enterprise (NX-01) periodically would receive a signal from an unknown origin. Originally the signals were unable to be decoded due to the primitive technology and were thus noted as an oddity but filtered out to allow for needed communications to pass.
The signal itself is a repeating sequence of alpha-numeric characters, spoken aloud. At the start of every message is the identifier Echo Signal Alpha 1 Omega before the alpha-numeric sequence is spoken exactly 3 times with a 30 second pause between each repetition. Once the audio stops, the carrier wave continues for exactly 5.29 seconds and then the carrier wave itself is terminated.
Sequence
This signal always repeats the same sequence of numbers and letters.
Two Four Nine Bravo Tango Five Nine Five Twenty-two Charlie Seventy Zebra Two One Two Nine Oscar Papa Romeo Nine Nine Seven Five Nine Eleven Forty-seven Tango Foxtrot Thirty-nine
Repetition
The signal occurs at regular intervals of exactly pi days at a given location. Though the exact time of each interval is not the same in every location, though every location receives on the exact interval of pi days from the last reception.
Historical Significance
Historians belive that the signal itself is analogous to the numbers stations which were heard often in the 20th and 21st centuries on Earth using conventional radio. These were often believed to be military or covert messages encoded for intended recipients to receive new orders or important information while unable to establish a more conventional channel of communication. Since the signal was first picked up by Starfleet on December 8th, 2155 by the Echo Two relay station and was also unable to be decoded without more advanced signal decoding algorithms first developed in the 2209 and not in military use until 2211 let alone civilian use until 2215 it is unlikely the signals comes from an Earth or Starfleet source.
This and other inconsistencies with conventional wisdom have prompted many to suspect that the early numbers stations from Earth history are somehow related which has sparked considerable speculation as to the nature of the signal and others of its type.
Investigations
Once the signal was decoded and heard to be a message in english, large amounts of research funding was spend by Starfleet Intelligence to find the source of the signal. It was determined that the signal is identical to the original signal first recorded and analyzed. Research into records from space faring organizations which have been using subspace radio for much longer also concluded that the signal was heard prior to 2155 but suspected to be background noise from an unknown source.
Using data from the Vulcan Science Academy along with data from the Andorian government, Klingon military database, and the Betazoid government a record of the signal occurring was constructed to begin around 427 B.C.E. (in Earth years) discovered by Andorian scientists who studied the signal but found it to be meaningless. While this is the first confirmed and recorded occurrence of the signal there is evidence of its existence before that, though records do not exist that confirm this.
Controversy
In the last 23rd century there was considerable controversy over whether or not the signal that was heard originally was the same one being heard now. It took another 15 years to develop a method of detecting if the signal was indeed the original signal being broadcast anew or a recording, cleaned up, and rebroadcast or a fake entirely. In 2389 the signal was analysed using complex computer modeling algorithms developed by the Daystrom Institute for military purposes. These algorithms were able to detect the difference between transmitters even if the signal itself contained identical information up to a 98.7% accuracy.
While some 22% of the recorded signals from just 23rd century were discovered to be fakes, all the other instances are from the same transmitter.