Female inmates at a Singapore prison are working 12-hour shifts as telephone call-centre operators and telemarketers in a state campaign to rehabilitate lawbreakers, an official said Wednesday.

"It's pretty much the same as a commercial call centre, except it's behind bars," said Vincent Chan, a senior manager at the Singapore Corporation of Rehabilitative Enterprises.

"It's our way of upgrading the old prisons' industries and enhancing the inmates' employability," Chan said.

He said the call centre is a cubicle-filled room about the size of a basketball court at the Changi Women's Prison and Drug Rehabilitation Centre.

The duties of the 38 inmates working there include answering questions about prepaid mobile phone cards and consumer products, he said, adding that supervisors monitor the calls to make sure they are limited to business.

The operators are trained to speak clearly and to soothe difficult customers.

The call centre operates around the clock and has 10 clients, including a telecommunications company, Chan said. Clients did not want to be named due to concerns that links with a prison could hurt their business.

Chan declined to say whether the prisoners are paid for their work, or to give any other details about the programme's finances.