EE offers wi-fi call back-up service
EE is to begin switching some of its customers to wi-fi enabled calls to help combat mobile signal dead spots.
The UK network suggests the move will particularly benefit people who fail to get a connection or experience dropped calls in their homes and offices.
Other firms already offer a similar service via apps, but EE says its scheme is "seamless" as users are not required to do anything to switch between 3G/4G and wi-fi.
However, there are potential pitfalls.
EE said its Wi-fi Calling facility would initially be limited to pay monthly subscribers using Samsung's Galaxy S6 and S5 phones and Microsoft's new Lumia 640, although the BBC understands the iPhone 5S and newer Apple handsets will also be supported.
Since it requires specific mobile data components to be built into the devices, it cannot be extended to other older models. However, EE said it would soon offer a compatible own-brand budget smartphone.
To join the service, users send the firm a single text message.
"We have worked more than a year to make sure that everything works like a normal phone connection," Olaf Swantee, EE's chief executive, told the BBC.
"So, the ringtone, the voicemail, the quality of the conversation - all of that is exactly the same with our solution.
"I think that's essential as customers don't have the time to figure out whether they should be using an app.
"They want to simply know the network will switch from one technology to the other without customer intervention."
He added that he expected five million people would be able to use the service, which also supports text messages, by the summer.
Even if a personal wi-fi network is used, the phone's owner will still be billed as if they had made the call in a normal manner. Furthermore, they cannot use it to make cheaper calls from abroad.
But one industry watcher said the scheme still had appeal.
"One of the complained about factors with mobile phones is just making a voice call, and perhaps an approach of getting the fundamentals of connectivity right can act as a differentiator for the firm," said Kester Mann from the telecoms consultancy CCS Insight.
"Vodafone will also be launching a similar wi-fi calling service in the summer."
The UK's other two leading networks, O2 and Three, offer apps instead: Tu Go and Three inTouch.
Third-party services, including Skype, Whatsapp, Viber and Apple's iMessage, also provide ways to make voice calls over wi-fi.
One of the limitations of EE's Wi-fi Calling is that it cannot switch from one service to another mid-call.
So, if someone walks out of a wi-fi covered zone into an area where there is 4G coverage, or vice versa, they still face their call suddenly ending.
EE is marketing the service as a way to make calls in the home or office, where it believes this is unlikely to be a common problem.
It also boasts that it will make it easy for people to make and receive calls on London Underground platforms, and other wi-fi enabled places without network coverage.
However, it acknowledges that users many need to manage their experience.
Because phones will automatically switch to wi-fi where it is available, if users log into a congested hotspot they could face poor call quality. They would then need to go into their settings menu to temporarily turn off the wi-fi connection or Wi-fi Calling facility to opt back into 3G/4G.
"It's going to depend on the quality of the public wi-fi, and if that's a poor quality service in a coffee shop, shopping centre or wherever, that will be a challenge," said Mr Mann.
"It's very much tailored to the private wi-fi that people have in the basements of flats, rural homes and other places with poor mobile coverage."
EE recently beat other networks in terms of average 4G download speeds and the number of UK premises with coverage, in a study carried out by the regulator Ofcom.
The company - which is currently owned by Deutsche Telekom and Orange - is in the process of being taken over by BT, which plans to let customers boost reception within buildings via a separate technique involving additional radio spectrum that it owns.
There is, however, opposition to the acquisition.
Consumer rights advocate Which? has written to Ofcom highlighting that the two firms have fared poorly in customer satisfaction surveys, and suggested that the merger could exacerbate the situation.
However, Mr Swantee insisted customers would benefit from the tie-up.
"The purchase of EE for £12.5bn is very much going to further innovation," he said.
"In-market consolidation can really help to drive investment.
"And when you look forward to technologies like 5G - which we will get in 2020, maybe - they require incredible investment."