Night or day, Mr David Loh can expect to receive an SMS asking him to turn up at a police station within 1½ hours to "aid in an investigation".

But hardened criminal he is not.

Mr Loh, 68, a retiree, is one of 208 volunteers in the Appropriate Adult Scheme. Under the scheme, launched in March last year, a trained volunteer accompanies a person with intellectual or mental disabilities during police interviews.

So when the police need to interview such a person, a request for help is sent out to the pool of volunteers. One of them will head to the police station. This volunteer then helps in the interview process so that the person being questioned by the investigation officers can respond more effectively.

Since the scheme was launched, the number of volunteers has grown from 60 to 208. This year, the administration of the scheme was taken over by the Movement for the Intellectually Disabled of Singapore (Minds), from the Law Society's Pro Bono Services Office. Between Jan 1 and the end of July, there were 136 Appropriate Adult activations.

Mr Loh, who has been a volunteer since the scheme was a pilot at Bedok Police Division in 2013, has responded to 31 cases since last year.

His assistance at police interviews ranges from simply checking with the person if he needs to use the toilet or is hungry, to helping officers interpret answers.

"There was an accused with low IQ, he refused to cooperate. When the officer asked what time he was at the scene, he made a flying noise with a hand signal, then put out four fingers," said Mr Loh.

The officer and Mr Loh could not understand what he was trying to say. Mr Loh asked the officer if they could get the person's father to come in to interpret the signals. It turned out that he had been telling the officer about taking a plane on a recent holiday with his family of four.

The job of the volunteers is to make sure misunderstandings do not occur, and that suspects do not admit to crimes they did not in fact commit, out of stress or misunderstanding. So far, volunteers and police have positive feedback about the scheme, said Minds chief executive Keh Eng Song.

Volunteers have to attend a one- day training session conducted by lawyers and psychologists, who teach them about the role of an Appropriate Adult - who has to remain neutral and not advocate for either side - basic police procedures, and how to communicate effectively with people with different disabilities.

People can sign up as volunteers, provided they are above 21 years old and they pass police screening. Lawyers and police officers, however, cannot sign up due to a conflict of interest.

Criminal lawyer Sunil Sudheesan, one of the trainers, said the scheme protects the integrity of police investigations when it comes to people with special needs, because a neutral third party is present.

"It is a measure which protects the investigation process, witnesses and investigators from baseless claims. And videotaping will ideally be introduced... so as to leave fact-finders with little or no doubt of what was actually said by suspects or witnesses," he said.

Meanwhile, Minds is looking to improve the scheme, especially in terms of volunteer training. It will also increase the pool of volunteers to 250 by the end of the year.

Mr Loh has a piece of advice for those looking to sign up: "You must really have the time to help. Don't just come in as an Appropriate Adult but when there is an activation, no one answers."