Some stories have happy endings, others not so.
Ouyang Guoqing, the executive director and chief cameraman of the show, says he was shocked by one of the cases when a patient who looked relatively stable suddenly died.
"The patient walked into the ambulance, and I thought there was nothing seriously wrong with him.
"However, things soon took a turn for the worse after we arrived at the hospital. I was dumbfounded when his life just ended in front of me.
"I was sad for a long time, and so were the doctors.
"But I later understood that the best way for medical practitioners to respect life is to get fully devoted to their next mission."
Sometimes, the situation was too urgent for the crew to inform the patients or their families that they were being filmed for a TV show, but Li says every story that features in the final production came with the families' consent.
It took the crew six months of observation to select nine front-line medical practitioners - eight male and one female - from more than 1,000 people, for the show.
In the show these "idols" have a tough life. Each of their shift typically lasts 12 hours, during which time an ambulance undertakes 13 to 15 missions.
In most cases, the medical practitioners with the ambulances also have to move the patients from their homes.
Wu Xin, 37, one of the medical practitioners shown in the TV show, says: "Unlike doctors in hospitals who can save patients in emergency rooms, every move we make is seen by the patients' families.
"And, sometimes, they may have different opinions about our methods, which can be a distraction," he says.
Li, however, says the show uses an objective tone to showcase the relationships between doctors and patients, a widely discussed topic in Chinese society. And it aims to help the general public better understand doctors.
Wu says the tough life of medical practitioners at emergency centers has made many of his colleagues leave the profession. And he hopes that this show can change perceptions.
Shanghai has almost 800 ambulances. In 2017, the ambulances in Shanghai went out 760,000 times on call.
Speaking about Shanghai's medical infrastructure, Wu says: "In terms of ambulance facilities, Shanghai is on par with the United States. But our expertise needs to continuously improve."
The show also features some embarrassing moments.
For example, some people even call 120 for a headache.
"A granny calls 120 because her maid is asleep, and she can't wake her up," says Wu.
In some cases, people call 120 for fun.
"So, we want to remind the public that this emergency service should be used only by those who really need it," says Li.
Meanwhile, On Call 120 is also scheduled to cover other Chinese cities in subsequent seasons.
