(Adds North Korean confirmation of Jang's removal)
By James Pearson and Ju-min Park
SEOUL, Dec 9 (Reuters) - As a scratchy rendition of Beethoven's Piano Sonata No 8 fades into a sea of shortwave radio static, a robotic female voice starts speaking in Korean.
"Number 1913, number 1913, incoming message," the voice says, before reading out seemingly random sets of numbers.
"68360, 75336, 80861, 94409, 03815," it continues in an eerily authoritative tone.
The broadcast, a method of sending one-way secret messages to spies, dates back to the French Resistance in World War Two and is still in use on the Korean peninsula, where human intelligence remains the most important way of gathering information.
Blanket electronic surveillance and satellite imagery offer only limited penetration in isolated North Korea, where the use of mobile phones and the Internet is far below global standards. But reliance on antiquated methods and human sources has meant that the National Intelligence Service (NIS), South Korea's spy agency, has a patchy record on finding out what is going on in nuclear-armed and unpredictable North Korea, with which it is still technically at war.
The agency scored a coup last week, however, by informing the world that Jang Song Thaek, the powerful uncle of North Korean leader Kim Jong Un, had been removed from his positions.
North Korea's official KCNA news agency confirmed Jang's sacking on Monday, saying he had been dismissed for what it described as a string of criminal acts including corruption, womanising and drug-taking.
It is the biggest upheaval in Pyongyang's secretive ruling circles since the sudden death of former leader Kim Jong Il in 2011 left control of the dynastic state and its 1.2-million-strong military in the hands of his young and untested son.
"For cases like the dismissal of Jang Song Thaek or events within the (North) Korean Worker's Party, we need human intelligence," said Yeom Don-jay, a retired NIS veteran.
"Humans know the minds of humans best," said Yeom, who worked at the spy agency for three decades before retiring in 2004 after postings in Germany, Brazil and the United States.
While the NIS got wind of Jang's dismissal, the agency's critics say it has struggled to get solid information on what is happening in the North for years, including the death of Kim Jong Il.
Some also say its disclosures may be aimed at diverting attention from an investigation of the agency for getting involved in the 2012 presidential election.
"It is widely known that there is a political motive behind (the disclosure). It is like a person is trying to hide his weak spot by saying 'I am in the know'," said Kim Yeon-chul, a professor of unification studies at Inje University.
Earlier this year, as President Park Geun-hye came under pressure to reform the NIS after charges that its agents had tried to influence the December 2012 vote in her favour, the agency released previously secret transcripts of a 2007 conversation between Kim Jong Il and his South Korean counterpart at the time, Roh Moo-hyun.
The agency has been notoriously averse to any attempts at reform over the years and its current chief, Nam Jae-joon, was quoted as saying the reform plan would leave its hands tied.
The details of the conversations embarrassed Roh's supporters in the opposition Democratic Party, who have been vociferously demanding reform.
The radio messages have been used by the South for decades, say sources with knowledge of how the country's secret agents operate.
"It's classic - the safest way to deliver messages, it leaves no trace," said former agent Yeom.
The messages work by sending strings of seemingly random numbers over shortwave radio signals to an agent in the field, armed only with a radio, pen and an easily concealed pad with corresponding letters on it that can be used to decrypt the messages.
"The first time I heard the South Korean numbers station now known as V24 was probably in the early 1980s," said a radio hobbyist who only identifies himself by his call sign, 'Token'.
Long-time listeners like Token say V24's unique power signature and signal strength place its origin somewhere south of the Demilitarized Zone that separates North and South Korea.
An official at the NIS who deals with media requests said he could not confirm anything related to the operation of South Korean numbers stations.
But hobbyists say the secret station is being used less frequently.
"In July 2013, I received 62 messages, most of them in the first half of the month," said Token, who monitors the signals from his location in the Mojave Desert in the United States. "However, in the first ten days of November, I only received three.
"I have never seen traffic anywhere near this low. This station could be winding down operations," he added.
A former South Korean commando who has worked behind North Korean lines said he also communicated with his handlers by radio and number codes.
"We had a morse code machine and a number pad. North Korean agents were using the same methods at the time," said the commando. "It still looks like a safe way (to communicate) now too."
But as isolated North Korea slowly modernises, and mobile phones and using the Internet become more common, the South's spy agency may have to adopt more up-to-date methods.
"Clearly North Korea has also been using a greater variety of communications technology, so it gives the South that many more channels for gathering intelligence," said a former South Korean official who worked in national security.
North Korea's growing thirst for mobile phones also means that strangers with expensive-looking technology might not attract the kind of suspicion they may once have.
"There are special mobile phones that the NIS uses that can't be traced. I think intelligence on Jang was likely received by that way - using people," said the former South Korean commando.
The NIS is housed between leafy mountains and the tombs of dead Korean kings in a sprawling cluster of nondescript office buildings, a short drive away from Seoul's glitzy Gangnam district.
Although it is air-brushed from satellite images and absent from online maps, its existence is no secret in South Korea.
For months, the agency has been the centre of a domestic scandal - prosecutors in Seoul said last month that 1.2 million tweets were sent by NIS agents during the 2012 presidential election in an attempt to boost the popularity of Park, the eventual winner and South Korea's first female president.
She is the daughter of assassinated president Park Chung-hee, who was killed in 1979 by the head of the intelligence service, then called the Korea Central Intelligence Agency.
Despite the agency's shady overtones, joining the NIS is still seen as an attractive job by the young.
"The quality of information from intelligence officers keeps getting better and better as it is now one of the popular jobs for young people," said a former agent who worked at the NIS until the late 2000s and declined to be named.
"We have talented human assets and Jang's case proves it." (Additional reporting by Jack Kim; Editing by Raju Gopalakrishnan and Dean Yates)