(Reuters) -As mobile phones became the world's main communications tool, pagers, also known as beepers because of the sound they make to notify users about incoming messages, were largely rendered obsolete, with demand plunging from their 1990s heyday.

But the tiny electronic devices remain a vital means of communication in some areas - such as healthcare 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 UK 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."

Pagers grabbed headlines on Tuesday when thousands used by members of militant group Hezbollah were detonated simultaneously across Lebanon, killing at least nine people and wounding nearly 3,000 others.

According to a senior Lebanese security source and another source, explosives inside the devices were planted by Israel's Mossad spy agency.

The UK's NHS was using around 130,000 pagers in 2019, more than one in 10 of the world's pagers, according to the government. More up-to-date figures were not available.

Doctors working in hospital emergency departments carry them when they are on call.

Many pagers can also send out a siren and then broadcast a voice message to groups so that whole medical teams are alerted simultaneously to an emergency, a senior doctor in the NHS said. That is not possible with a mobile phone.

Britain's Royal National Lifeboat Institution (RNLI) uses pagers to alert its crews, a source familiar with the lifeboat service told Reuters. The RNLI declined to comment.

PAGERS HARDER TO TRACK

Hezbollah fighters have been using pagers as a low-tech means of communication in an attempt to evade Israeli location-tracking, two sources familiar with the group's operations told Reuters this year.

Pagers can be harder to track than smartphones because they receive messages transmitted via radio signal, while mobile phones send information to the network to find the nearest cellular tower and stay connected, allowing it easier to trace.

Pagers also lack more modern navigation technologies like the Global Positioning System, or GPS.

These have made them a popular choice among criminals, especially drug dealers in the United States, in the past.

But gangs are using mobile phones more these days, former FBI agent Ken Gray told Reuters.

"I don't know if anyone uses them (pagers)," he said.

"They all went to cell phones, burner phones" which can be easily disposed of and replaced with another phone with a different number, making them difficult to trace.

Gray, who served 24 years at the Federal Bureau of Investigation and now teaches criminal justice and homeland security at the University of New Haven, said that criminals changed with the times and newer technology.

The global pagers market, once a major source of revenue for companies like Motorola, amounted to $1.6 billion in 2023, according to an April report by Cognitive Market Research.

That amounts to a tiny fraction of the global smartphone market, which was estimated at around half a trillion U.S. dollars as of end-2023.

But demand for pagers is rising as a larger patient population creates more need for efficient communication in the healthcare sector, the report said, forecasting compound annual growth of 5.9% from 2023 to 2030.

It said North America and Europe are the two biggest pager markets, generating $528 million and $496 million in revenue respectively.