Maybe you could read the article and learn something:

But the tiny electronic devices remain a vital means of communication in some areas - such as health care and emergency services, thanks to their durability and long battery life.

“It’s the cheapest and most efficient way to communicate to a large number of people about messages that don’t need responses,” said a senior surgeon at a major U.K. hospital, adding that pagers are commonly used by doctors and nurses across the country’s National Health Service (NHS). “It’s used to tell people where to go, when, and what for.”

Smartphones do a lot of things that might not be needed (look into how many different sensors they have). Sometimes a person doesn’t have access to a charger or time to charge a device and running out of battery could mean someone dies.