Suspects involved in telecom swindle cases are escorted off an aircraft by the police at Beijing Capital International Airport in Beijing, capital of China, Nov 10, 2015. [Photo/Xinhua]

"We were pressured every day," said a suspect, adding that if someone could not finish his task several days in a row, he might get fired.

Suspects were divided into teams, with managers at different levels supervising. Every fraud phone call was recorded, and managers commented on the records every night.

Suspects pretended to be staff of mobile companies such as China Unicom and China Mobile, banks such as Bank of Shanghai and Industrial and Commercial Bank of China, and staff of other financial companies,

Police discovered that the gang even used to pretend to be staff at sub-offices of Shanghai and Beijing Public Security Bureau.

Wei said: "recruiting (workers) in China and sending them to dens abroad" has turned into an "industrial chain".

Suspects had been recruited in China for highly-paid jobs abroad, and got visas in the southern city Shenzhen before being sent abroad. They were usually transferred in Hong Kong instead of flying directly to the destination to avoid being noticed by police.

As soon as they arrived they were met and sent to the dens. The suspects live in villas and start calling soon after brief training.

New members' passports are usually handed in, to keep better control and they are not allowed to use mobile phones, getting a small of money every month.

Yan Dongjie contributed to the story.

Contact the writer at yandongjie@chinadaily.com.cn.