British Transport Police, the division of the police dedicated to protecting the UK’s railways, has unexpectedly revealed they are also on the front line for fighting networked fraud. Last week they shared news of the prosecution of Kong Ji Chen (pictured), a fraudster who was caught using an SMS blaster to send smishing messages to passengers on the London Underground, the capital’s subway system. The press release began:
A man who used an SMS blaster to spam commuters with fraudulent text messages during rush hour has been jailed, following a British Transport Police (BTP) investigation.
Kong Ji Chen, 31 (09/10/1994) and of no fixed address, was sentenced to 24 weeks imprisonment at Inner London Crown Court yesterday (15 October) having pleaded guilty at an earlier hearing.
The court heard how on Tuesday 1 July 2025, at around 5.30pm, an alert member of the public received a spam text message while travelling on the Victoria Line.
The text claimed to be from Parcel Mail and invited the reader to click on a link to update their details in order to receive a package. Users who did so would then be asked to enter in their personal details.
The police were keen to draw attention to the crime. In addition to issuing the press release, British Transport Police shared video footage of the arrest via their official accounts on TikTok, Instagram and X. You can see their post to X below.
This is Kong Ji Chen, caught by BTP with an SMS blaster in his suitcase 🎒
He was loitering on the Underground, sending phishing texts to commuters. Thanks to alert passengers, he’s now serving 6 months in prison.
Spot something odd? Text 61016
🔗 https://t.co/1VaL9DR7Wr pic.twitter.com/RrDQDh8l58
— British Transport Police (@BTP) October 17, 2025
A commuter was praised for calling the police after receiving the smishing message and then realizing it was connected to Chen’s odd behavior.
The member of the public was familiar with SMS blasters and grew suspicious of Chen when he noticed him loitering on the platform with a large suitcase, not getting on any trains or moving on from the platform.
Local officers were alerted and arrived at Victoria Underground station, where Chen was found sitting on a bench next to a large green suitcase. Chen claimed he was waiting for a friend, and claimed the suitcase had been given to him earlier in the day by another man.
Officers searched the suitcase and upon opening it saw a portable battery attached to an electrical unit with a green flashing light and a black antenna. Chen was then immediately arrested and taken into custody.
Telcos received 165 separate reports about the same scam SMS message on the day of the arrest, with the first report logged at 2.38pm. Police who arrested Chen also received the same message on their own phones. Given the typical ratios for the number of people who report scams to the number of people who receive them, we can estimate that between 5,000 and 20,000 people received messages from Chen’s SMS blaster in the three hours before he was arrested.
I am disappointed in myself for not previously identifying the risk that subway commuters will be scammed using this technique. From the fraudster’s perspective, underground public transport is the best possible location for an SMS blaster.
- Criminals compensate for the restricted range of SMS blasters by moving them around places where many people congregate. That is why they drive them around city centers and carry them through shopping malls. But there is less need to move the SMS blaster if hundreds of different people are passing by every few minutes. A route known to be used by SMS blaster scammers in Tokyo runs alongside the two busiest train stations in the world. The London Underground line where Chen was arrested serves over 800,000 passengers during a typical day. It would be entirely predictable that messages sent from a platform in the heart of London would reach several thousand passengers per hour as they board, disembark, or simply sit on their train while waiting for it to move.
- Crooks like sending SMS messages for the same reason businesses do: recipients of SMS messages tend to read them immediately. It is widely claimed that SMS messages have a 99% open rate and that 90% of SMS messages are read within 3 minutes of receipt. If those figures are going to be surpassed in any specific location, it will be in places where bored commuters are standing alone with nothing to do but wait for the next train to arrive, or while they are sitting on the train, waiting for it to start rolling again. Passengers may be especially vulnerable when surprised by the novelty of receiving a message while underground because it is a place they might normally expect to have no network coverage.
- Being in a tunnel that inhibits the transmission of radio waves would generally be considered a disadvantage for radio communications, but not if you do not want those radio waves to be discovered by anyone outside of the tunnel. The prime method that network operators use to detect the existence of SMS blasters is to look for anomalous coverage gaps that occur because mobile phones are connecting to fake base stations instead of connecting to the operator’s base stations. That technique will not work in locations where the telco never has any coverage. I traveled on London Underground trains recently, and was pleasantly surprised because my phone remained connected to the network more than it ever had before, but I still had no reception for the majority of those journeys. The enormous network black spots created by underground metro systems are an ideal environment for SMS blasters to be used without any risk of them being identified by telcos, and with significantly reduced risk of them being located using specialized radio detectors.
The mistake that Chen made was that he did not move like a normal passenger moves. If he had kept shuffling around the subway system by boarding trains, riding them to the next stop, disembarking and then changing to a different line, it is difficult to imagine how anyone would have identified him as the source of the SMS messages. Even if somebody thought his bag contained an SMS blaster, they would be unlikely to stop and challenge him before he had boarded a train going in the opposite direction to them. Chen carried a large bag, which also drew attention to him, but it is not unusual to see people with luggage on the subway lines that service London’s airports. There are also smaller SMS blasters that are less conspicuous. Last year, two scammers from Hong Kong were arrested while carrying a backpack SMS blaster through shopping malls in Bangkok. Any regular passenger on London Underground will share stories of how often they have been buffeted by somebody else’s backpack. I find it hard to believe anybody would have caught Chen if he had been walking from train to train while carrying an SMS blaster in a backpack, so long as he ensured the device was only active while he was underground.
The impossibility of detecting a radio scam device operated in this manner, with the technology available to us at present, may explain why the British Transport Police have gone to such unusual lengths to publicize this case. It is probably not an accident that the press release mentions that the person who identified Chen’s suspicious behavior was ‘familiar with SMS blasters’. If passengers do not question the behavior of strangers standing or sitting alongside them then the chances of catching the next Chen will be close to zero. But we also do not want a scenario where foreign-looking tourists are confronted by mobs just because they are carrying a bulky bag. The authorities will face considerable challenges if criminal gangs keep replicating and refining the use of SMS blasters on underground public transport. They will not want to stoke panic or xenophobia, but they will also be loathe to admit that devising a cost-effective technological solution for detecting SMS blasters while underground is currently beyond their capabilities. This makes it even more important to criminalize the importation, supply and possession of equipment that can act as a base station unless there is a demonstrably legitimate purpose for having it. Some Brits reading about Chen’s arrest may now be regretting that SMS blasters were not outlawed in the same way that simboxes were prohibited by the Crime and Policing Bill passed earlier this year.
