When Sir Paul McCartney performed at the Santa Barbara Bowl, he promised fans an intimate gig. But the former Beatle went a step further than most by agreeing to engineer a makeshift "lockdown" on selfies and filming at the concert.

All 4,500 fans had to place their mobile phones in lockable pouches for the duration of the concert, and enjoyed the gig completely "phone-free".

"Nobody's got a phone," McCartney declared during his 25-song setlist. "Really, it's better!" he added.

Achieving a large-scale phone ban is a startlingly simple process.

On the way into a venue, concert-goers have to put their phones into a pouch which is magnetically locked.

They keep the phone on them, and the magnet releases at the end of the performance.

Artists such as Dave Chappelle, Alicia Keys, Guns N' Roses, Childish Gambino and Jack White have embraced the freedom saying it allows them to perform at their best - or even experiment more.

In an interview in Rolling Stone in June, Sabrina Carpenter discussed potentially banning phones at future concerts.

Some music lovers seem to be embracing the idea.

A fan at a Lane8 DJ gig, Shannon Valdes, posted on social media: "It was refreshing to be part of a crowd where everyone was fully present - dancing, connecting, and enjoying the best moments - rather than recording them."

For the man behind the pouch technology, his own Eureka moment similarly came at a music festival back in 2012.

"I saw a man drunk and dancing and a stranger filmed him and immediately posted it online," Graham Dugoni explains. "It kind of shocked me.

"I wondered what the implications might be for him, but I also started questioning what our expectations of privacy should be in the modern world."

Within two years, the 38-year-old ex-professional footballer founded Yondr, a US start-up that promotes phone-free spaces.

The lockable pouch market is still in its early stages, but more companies are starting to appear. The pouches are widely used in theatres and art galleries and increasingly in schools.

They cost between £7 and £30 each, depending on the supplier and the size of the order.

Yondr has worked with around 2.2 million schools in America and says around 250,000 children in England now use its wallets across 500 schools - including one academy trust in Yorkshire which has spent £75,000 on Yondr pouches.

Paul Nugent created Hush Pouch after working for 20 years installing lockers in schools. He says there's a lot for headteachers to consider.

"Yes it can seem an expensive way of keeping phones out of schools, and some people question why they can't just insist phones remain in a student's bag," he explains.

"But smartphones create anxiety, fixation, and FOMO - a fear of missing out. The only way to genuinely allow children to concentrate in lessons, and to enjoy break time, is to lock them away."

Yondr's Dugoni says school leaders have reported a number of benefits from adopting a phone-free policy.

"There have been notable improvements in academic performance, and headteachers also report reductions in bullying," he explains.

Vale of York Academy in York began using the pouches in November and headteacher Gillian Mills told the BBC: "It's given us an extra level of confidence that students aren't having their learning interrupted.

"We're not seeing phone confiscations now, which took up time, or the arguments about handing phones over, but also teachers are saying that they are able to teach."

Conservative leader Kemi Badenoch has said her party would seek to ban smartphones altogether from schools if it entered office.

The Labour government has stopped short of an England-wide ban on smartphones in schools, saying headteachers should decide, but has launched a consultation on banning social media for under-16s.

It is part of a series of measures that will also see England's education inspectorate, Ofsted, given the power to check policies on phone use when it goes into schools, with ministers saying they expect schools to be "phone-free by default" as a result.

Nugent says the feedback from parents is that most feel their child is safer having a phone on them while travelling to and from school, rather than leaving it at home altogether.

"The first week or so after we install the system is a nightmare," he adds. "Kids refuse, or try and break the pouches open. But once they realise no-one else has a phone, most of them embrace it as a kind of freedom."

The continuous expansion of social media platforms and AI brings the concept into direct competition with the San Francisco tech giants and their algorithms, which are designed to constantly promote the use of smartphones in everyday life.

But Nugent believes a societal pushback is gathering momentum.

"We're getting so many enquiries now. People want to ban phones at weddings, in theatres, and even on film sets," he says.

"Effectively carrying a computer around in your hand has many benefits, but smartphones also open us up to a lot of misdirection and misinformation.

"Enforcing a break, especially for young people, has so many positives, not least for their mental health."

Dugoni agrees we are reaching a crossroads.

"We're getting close to threatening the root of what makes us human, in terms of social interaction, critical thinking faculties, and developing the skills to operate in the modern world," he explains.

"If we continue to outsource those, with this crutch in our pocket at all times, there is a danger we end up undermining what it means to be a productive person.

"And that is a moment where it's worth pushing back and trying to understand where we go from here."

Those 4,500 McCartney fans singing along to Hey Jude in the late September sunset might feel he has a point.

Should smartphones be locked away at gigs and in schools?