'Super Voice' 4G service from Three offers better signal
Mobile phone provider Three has launched a UK service it says will improve reception inside buildings and in rural black spots.
Its 4G Super Voice enables customers to make calls and send texts using a lower frequency spectrum.
Other networks are looking into introducing the technology, known as Voice Over Long-Term Evolution (VoLTE).
It currently works on only the Samsung Galaxy S5, but recent iPhone handsets will be added in the coming months.
It is being rolled out in:
Three said up to 5.5 million customers would have access to the service by 2017.
Chief technology officer Bryn Jones said: "By the end of the year, one million of our customers will have access to better indoor coverage and be able to use their phones in more places than ever before."