As the world's largest mobile phone fair hosts the "creme de la creme" in Barcelona, activists slam the dark side of a sector accused of ignoring rights abuses in Chinese factories and Congolese mines.
"Behind this modern, progressive industry lurk dark themes like labour exploitation or minerals stained with blood that come from conflict zones," says Alba Trepat, campaigns officer for SETEM, a federation of 10 NGOs that promotes an alternative fair with a social twist.
Just a few blocks from the main Mobile World Congress (MWC), the so-called Mobile Social Congress has even showcased its very own superstar smartphone - the Fairphone 2, which a Dutch firm tries to manufacture as ethically as possible.
"Traceability is quite a difficult issue," recognises Daria Koreniushkina, public engagement manager at Fairphone.
"The supply chain involves hundreds of actors globally and just for one mineral you have at least five steps where the mineral goes.
"To make sure that every step of it is ethical is a very challenging exercise."
DRC, Peru, China
The firm's first model tried to use tin and tantalum from the Democratic Republic of Congo (DRC) that was not linked to the country's armed conflict.
For its second phone - launched in 2015 - Fairphone used gold extracted from Peru respecting fair trade standards.
In its Chinese factory, meanwhile, the company promotes social programmes for employees, bettering worker representation and publishing independent reports on labour conditions.
"We cannot magically find the fair factory in Asia but a first step is transparency and willingness of the factory to work towards improvements," said Koreniushkina.
According to Electronics Watch, a watchdog working to better labour rights in the global electronics industry, workers in many Chinese factories toil for more than 80 hours a week, with low salaries and high exposure to carcinogens.
In 2012, Apple even admitted that some of its suppliers had committed labour abuses after a wave of suicides hit Taiwan's Foxconn company, which makes components for the US giant.
The sector is lucrative. In 2015, it contributed $3.1 trillion (2.8 trillion euros) to world output, according to GSMA, organiser of Barcelona's main mobile phone fair.
"This industry would be the world's fourth largest economy," says Josep Maria Royo, a researcher at the Autonomous University of Barcelona who studies armed conflicts.
"They boast about having these resources, so they should be responsible."
Sunlight? What is that?
Royo is also part of a small, local NGO that denounces how the mobile industry indirectly finances armed conflict in the DRC, one of the main producers of cobalt and coltan - essential components for many smartphones.
"The DRC produces more than half of the world's cobalt," says Maria Canadas, head of Amnesty International in the northeastern Spanish region of Catalonia, where Barcelona is located.
"Some 20 per cent of the cobalt it exports comes from makeshift mines in the country's south, where workers excavate with their hands, without any tools or protection."
In January, Amnesty published a report denouncing conditions in these mines for workers - many of them children - who toil away from dawn to dusk, seven days a week, sometimes mistreated.
"UNICEF estimates there are around 40,000 children working in these mines," she says.
"Some of those we interviewed - as young as seven - said that they had not seen sunlight for a long time as they spent the day in the mines.
"I can't believe that the people at the MWC don't know this. But they're not interested in talking about it, and even less in resolving it."
Fairphone wants to become a leading player in the fair trade mobile sector.
Some 60,000 people in Europe bought its first model, and 30,000 units of its second phone have been sold so far.
"By showing there is a demand for more ethical products, we can inspire the whole industry to act more responsibly," says Koreniushkina.
"And maybe one day Mobile World Congress and Mobile Social Congress are going to become synonyms."