The Estonian sigint agency monitored Soviet traffic during the 1930’s and cooperated with the similar departments of Germany and Finland. Unfortunately it is very difficult to find information on their operations and successes.Some information is available from the very interesting article ‘Estonian Interwar Radio-Intelligence’ by Ivo Juurvee (Baltic Defence Review No. 10 Volume 2/2003) , uploaded on site bdcol.ee
Some quotes:‘The Estonian pre-war military intelligence service - the Second Department of the General Staff - and especially its radio-intelligence branch, Section D, have not been researched much…’
‘The Wireless Station of the General Staff in Tallinn intercepted the first radio messages of the Red Army during the War of Independence (1918-1920).’
‘In contrast to other parts of the Second Department, the personnel of Section D as of summer 1940 is precisely known: it was 26 people . two officers, 23 NCOs and one private. Nobody had been hired before 1936. This confirms the supposition that Section D was formed in 1936-1937. The second officer, Olev Õun, was taken to service only in March 1938; so far Andres Kalmus had managed to supervise the section alone. Radio-intelligence had gone through two major enlargements. The first of them was at the beginning of 1937, when Section D had just started its work. The second occurred in summer of 1939, when, according to President Konstantin Päts. secret decree from July 10, .due to complex situation [in Europe] naval radio intelligence has been reinforced.. With the order of the Commander-in-Chief General Johan Laidoner from July 22, the radio crew of the Second Department was enlarged .substantially..’The top codebreakers were Andres Kalmus and Olev Õun. Note that these names also show up in some TICOM reports.
‘Captain Kalmus had followed military radio courses abroad.’‘Olev Õun was especially talented, who was, in Hallamaa’s opinion, a phenomenal decipherer and had managed to break the latest code of the Red Army during the Polish campaign in September 1939.’
‘In 1939-1940 Section D units were stationed in Merivälja (7 km to the East from the city centre of Tallinn, probably next to the lighthouse of Viimsi, where the post of Naval Communications was situated, or somewhere in the area of nowadays Ranniku Road or Mõisa Road), Narva (probably at Olgino Mason 5 km to the North-East from city centre) and Tartu (probably in some of the units of the 2nd Division).’
‘When the Second Department closed down, it handed 51 items of literature over to the Red Army, including nine items concerning cryptology, a Russian-Estonian military dictionary and three Krypto ciphering clocks.’