News of the World investigator hacked into ATM landline near where he lived, and used it to access stars' voicemails

Police were astounded to discover at one point that Glenn Mulcaire secretly hacked into a cash machine in a convenience store in Sutton and used its phone line to access the voicemails of the rich and famous.

On the day that the News of the World's specialist phone-hacker was in court for the start of his sentencing hearing, having pleaded guilty months earlier, the Guardian can now reveal some of the lengths to which he went to conceal his illegal activities.

The cash machine hack was uncovered by police investigating Mulcaire in 2006, and was referred to in his sentencing hearing in 2007 after he pleaded guilty to phone hacking. It was not referred to in this year's trial.

"The sole purpose of that telephone line was to dial into the banking system when the cashpoint was in use. In other words, it was not really a telephone at all. It was simply a line to carry data to a cash machine to a database within the banking system, so as to record transactions carried out on the machine," prosecutors told the court at the time.

Alarm bells started ringing when the company that had registered the machine, Cash Point Machines, began to get bills from BT. The company forwarded the bills to the owner of the shop, located on a scruffy terrace of retailers on an approach road to Sutton, south London, near where Mulcaire lived.

Detectives found from itemised bills that the landline in the cash machine was being used by Mulcaire to hack the phones of members of the royal household and other famous people. Those working on the case said the shopkeeper was in no way implicated.

Mulcaire was so brazen about his activities that he even called BT in January 2006 to complain about a fault on the line on the cash machine.

While Mulcaire was charged and jailed in 2007, it was not until the phone-hacking investigation was reopened in 2011 that the "industrial scale" of Mulcaire's activities was finally acknowledged.

His office, in an industrial estate in Sutton, was like a James Bond set with three phone lines for hacking and walls lined with whiteboards setting out his targets. The phone-hacking trial heard that the total number of victims was 5,500.