A police clinical psychologist says telephone scammers are highly-trained professionals, who use psychological techniques to assert authority over their victims and isolate them. Woo Chin-pang urged people to be vigilant against scams, saying they were becoming more prevalent in Hong Kong.

Woo said once scammers had engaged in conversation, they were very good at using psychological concepts like establishing authority, credibility, social isolation to lure their victims. He said these kinds of techniques and concepts are often seen in sales and marketing techniques.

"All those scammers would like to introduce themselves and by establishing their authority by saying they are the officials from the police, or from China, or from other countries - or they are from the CHP [Centre for Health Protection] and so and so forth," he said. "And they will try to use the social isolation to trick the victims into believing they are in urgent danger or need to prove their innocence.

Woo also warned people to be suspicious of the number +852 appearing on their caller display - as this meant the caller wasn't in Hong Kong. The +852 code is the international dialling code for Hong Kong but it shouldn't appear on a Hong Kong number display when in Hong Kong.

On Wednesday, police said they had recorded more than 900 phone scams in the first seven months of the year. That's up 60 percent compared to the same period last year.