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."
Mobile phone provider Three has launched a UK service it says will improve reception inside buildings and in rural black spots.