AT&T Inc. is breaking a long-standing ceasefire among incumbent telephone carriers, by offering its CallVantage VoIP service on a trial basis, in markets traditionally served by Verizon and Qwest.

The little-publicized internet phone service will be sold in 14 AT&T Wireless stores in central New Jersey and Portland, Oregon. It will be bundled with cell phone service at a discounted monthly rate of $19.99.

This puts CallVantage in direct competition with the landline telephone services of Verizon Communications in New Jersey and Verizon and Qwest in Portland. If successful, the service could be extended to other markets outside of AT&T’s mostly Midwestern and Southern U.S. territory.

“We want to get a sense for how this sells,” explained AT&T spokesman, Michael Coe. “It’s something that we will look to potentially roll out more broadly.”

American landline incumbents have generally avoided stepping onto one another’s turf in the voice sector, even though such competition has been theoretically possible since the telecom industry was deregulated in the 1990s. As carriers jostle for dominance in the wireless sector, however, and the benefits of bundled services become increasingly apparent, one analyst predicts that AT&T and the other telecom giants will move towards “an inevitable future” of head-to-head voice competition.