AT&T will introduce a wireless voice service with clearer, more realistic sound next week in the Twin Cities and parts of three other states, a development that signals a big change in cellphone networks.

The new service, called High-Definition (HD) Voice, uses a wider range of frequencies to improve the sound quality of calls, said Mark Giga, an AT&T spokesman. It also provides a noise-cancellation feature that shuts out some background noise, making it easier to hear and be heard on the phone.

The service starts next Friday and will also be offered in Chicago, Milwaukee and other cities in Illinois, Indiana and Wisconsin, Giga said.

Consumers will need a new phone to use the HD Voice service. The first of those phones is the new Samsung Galaxy S4 Mini, but others will soon follow.

The HD Voice service won't cost anything extra, although customers will have to sign up to use it, Giga said. The price of the new Samsung phone isn't available yet.

Verizon Wireless plans to offer a similar HD Voice service later this year, said Karen Smith, a spokeswoman. Some existing Verizon phones will be able to be upgraded to use the planned HD Voice service, she said.

Behind the introduction of HD Voice lies a major shift in the cellphone system, one made possible by the introduction of 4G, or fourth-generation, cellular networks in the U.S. beginning in 2010.

Since 4G was introduced, AT&T and other cellular service providers have actually been operating two wireless networks at once: the traditional cellular network that carried voice calls and text messages for new 4G phones (and all communications for older 3G and 2G phones) and the 4G network that carried only the data for 4G phones.

But AT&T's new HD Voice service will change that balance: It will route voice calls over the 4G data network, using the same VoIP (voice over Internet Protocol) technology that is familiar to consumers who have used services such as Vonage or Skype.

As consumers upgrade to new phones that can use these HD Voice services, more voice calls will become permanently shifted to the new network, along with text messages that are really just data anyway. In the long run, the traditional cellphone networks are likely to become unnecessary, although that could take years because there are still many 2G and 3G cellphones in use that depend on the traditional networks.

"We have no plans to phase out 2G or 3G phones," Giga said.

Smith said Verizon Wireless would continue to maintain its traditional network for 2G and 3G phones while investing in its 4G network.

HD Voice won't be the only new service accelerating the shift to the 4G network. Videophone service for smartphones will be introduced by AT&T and Verizon Wireless later this year. And, like HD Voice, the video service will work only on the 4G network, which has the extra capacity to accommodate it.

In addition, AT&T is counting on its 4G network capacity to provide better voice and video quality than competing smartphone services such as Microsoft's Skype or Apple's FaceTime, Giga said.