This answer to the Chinese Language SE question Historical: How was Chinese written in telegrams and early computers? says (in part):
Following is a telegram code "dictionary" used by Japan military that already decoded the Chiang Kai-shek KMT army telegram code during World war 2. Even USA military know about such "open code", but USA never notify Chiang Kai-shek about the issue of leaking military secret, because USA itself want to know KMT army movement.
and contains the image below.
Question: How frequently did the KMT used telegrams to communicate, from where (roughly) and to whom?
As pointed out in comments (1, 2) the data may never have been considered particularly "secret" as the code book in the image was used primarily to change Chinese characters encoded as numbers (a form transmittable via telegram; a method of endoding) into Japanese. See also Ciphers vs. Codes.