Because the prisoner knew that Andy Lau was sent by Sam, that is, one of his own. After dialing the number, telling Andy Lau to get out was actually a signal to the person on the other end of the phone to run away.
The role played by Andy Lau is a senior inspector of the Criminal Intelligence Division and Han Chen's insider. Liu Jianming grew up in a housing estate. He joined the underworld when he was young and ignorant, and later joined the police force under the guidance of the underworld boss Han Chen. After years of hard work in the police force, and with Han Chen's support, he was able to provide a large amount of intelligence on the enemy's underworld crimes and was promoted to a senior inspector of the police force.
So, the prisoner can be covered to report the news.
Extended information:
The story outline of Liu Jianhua as an undercover policeman and Chen Yongren as an undercover gangster
In 1991, 18-year-old triad member Liu Jianming (played by Andy Lau) ) followed the instructions of his elder brother Han Chen (played by Eric Tsang), was admitted to the police academy and became an undercover policeman.
At the same time, another student at the police academy, Chen Yongyan (played by Tony Leung), was arranged by the police to drop out of school. On the surface, they forced him to drop out of school, but in fact they asked him to join a triad as an undercover agent.
After graduating from the police academy, Liu Jianming entered the police force and rose through the ranks, becoming a member of Team A of the Criminal Investigation Department. During this period, he used various opportunities to provide Chen Han with a large amount of intelligence.
Chen Yongren has also gained Han Chen's initial trust over the years. However, because Han Chen's case has never been solved, he can always associate with the gang. Only Officer Huang (played by Huang Qiusheng) has a single line of contact with him. .
Baidu Encyclopedia - The Departed