AT&T Corp. announced yesterday that it has started selling a service that sends local and long-distance phone calls over the Internet to residential customers in Washington and cities in 10 states.

The largest long-distance carrier has been making a big push to sell the new technology as an alternative to conventional phone service from local providers such as Verizon Communications Inc.

In the Washington area, AT&T said its Internet phone service is available in the District, Arlington, Falls Church, Fairfax, Alexandria, Herndon, Montgomery County, Prince George's County and parts of Southern Maryland.

"We said we'd be in 100 markets coast to coast in 2004, and so we've been on the mark since we launched 14 weeks ago," said Gary Morgenstern, a spokesman for AT&T.

The new markets are in addition to service initiated in recent months in cities including New York, Chicago, Dallas and Los Angeles. "We've really been pushing hard to bring this to market,'' Morgenstern said. "It's a good competitive service for households that are ready for that change."

Using the service requires a high-speed Internet connection, usually a cable modem or a digital subscriber line. The customer uses a device provided by AT&T to connect a conventional telephone to a computer modem. Although the phone calls are carried over the Internet, the caller can reach any phone number.

AT&T's CallVantage service includes additional features like caller-identification and a form of call-forwarding that allows up to five phones to ring at once.

AT&T is offering the service at a discount for the first six months: $19.99 a month for unlimited local and domestic long-distance calling. After that, the service costs $34.99 a month.

Companies such as Vonage Holdings Corp., which sells a similar service, have helped established the voice-over-Internet protocol, or VoIP, technology. Vonage has signed up 195,000 customers since it launched in March 2002. Verizon has said it will roll out its own version of the technology as an alternative to conventional service.