Suspects involved in telecom swindle cases are taken off in a bus by the police at Pudong International Airport in Shanghai, east China, Nov 10, 2015. [Photo/Asianewsphoto]

Nearly 50 people suspected of phone fraud were extradited from Indonesia to Shanghai on Tuesday.

A joint operation between Indonesian police and Chinese opposite numbers from Shanghai, Beijing, Guangdong and Taiwan was launched in September after several phone fraud dens were discovered.

China's Ministry of Public Security arrested 254 suspects in Jakarta and Phnom Penh, Cambodia, Shanghai Police arrested 47 among the 254, destroying three cells. They returned them to Shanghai from Jakarta.

"Shanghai police have been fighting transnational phone-fraud crimes on a large scale in recent years," said Bai Shaokang, deputy mayor and director of Public Security of Shanghai.

Shanghai Police trawled through more than 45,000 phone calls from Indonesia since July, uncovering 453 suspected victims. By talking to them police identified clues leading to the suspects' locations in Indonesia.

Wei Jian, a deputy division leader of Shanghai Public Security Bureau, said the den he was most impressed by was in Surabaya in Indonesia.

A four-floor villa covering an area of more than 400 square meters had many steel cots for workers on the second floor.

A suspect said that they worked from 8 am to 5 pm, and each had to hand in the names of at least 150 potential victims every day. Monthly salary was 10,000, depending on performance.