Ambitious plans will also see 7.5 million using VoLTE in 2016.
Three has announced a series of major upgrades to its network, taking place later this year.
Among the innovations, its existing network will gain low frequency spectrum in September, enhancing Three’s coverage footprint.
Three will also spread add Voice over LTE (VoLTE), allowing voice services to run on its 4G network. Currently Three’s 4G only carries data services, but the operator plans to have 1 million using VoLTE in 2015 and 7.5 million in 2016.
Bryn Jones, Three’s Chief Technical Officer, said: "Our aim is to keep our customers connected wherever they are. Adding low frequency spectrum to our network and enabling voice calls over 4G and wi-fi, will mean Three customers can stay connected in more places than ever before."
He continued: "With roaming at no extra cost also available in 18 destinations, Three is doing more than any other network to give our customers a reliable, quality experience wherever they are."
Three also revealed that over 1 million customers have now downloaded the Three InTouch app which provides the ability to call and text over wi-fi. Rival communications provider EE has also recently announced its wi-fi calling feature.
Danny Dixon, Director of Customer Strategy, said: "The Three inTouch app has been highly successful with over 1million users to date. Seamless connectivity on mobile and WiFi is a key part of our future strategic direction and by using the app, our customers will never miss that important call or text, whether they’re in the home, commuting to work, or away on holiday."