Caliope Sends: Probable One-time pad number cypher. Each group of five repeated for clarity. Length of total message stream is not necessarily significant, since this type of cypher system inserts "nulls" to fill out a message block. Also, know that a five-character set could consist of a single letter or a lengthy phrase. If this is a "book-cypher" certain five-character groups could point to a page or paragraph in the book. If a one-time-pad, each group would define a character within the pad. Look up "one time pad", "book code", "book cypher" and "numbers stations"

Be aware the the transmitter site may not be in the country or region of the actual message originator. Also, know that the voices used to speak the numbers may use accents different from the originator. Slavic accent used to mask identity of Spanish speakers is an example. This stream is voiced by an artificial computer voice designed to mimic a Slavic (possibly Ukrainian) accent.

Morse code numbers stations use variable types of Morse, including International Morse Code, Cyrillic Morse, American "railroad" Morse, and oriental character block/book codes.

More sophisticated over-the-air systems are using error-correcting data streams, such as FT8, etc., on split frequencies or hopping frequencies, there are also examples of spread-spectrum transmissions. Finally, use of "dead" or "retired" earth satellite radio channels are used, mainly the single-channel "order wire" which even decommissioned satellites leave open.

Do research and you will find this whole topic fascinating then listen on the "life-line" frequency for exo-solar system variations. "Life-line" refers to the 1200 gHz "water-hole" frequency. Lots of Ham operators are into to all of this stuff.