A herd of bison grazes in the Lamar Valley of Yellowstone National Park in Wyo.

Billings, Montana - Adventure seekers encounter untamed wilderness when they enter the backcountry of Yellowstone National Park.

Howling wolves. Deadly grizzly bears. Steam-spewing geysers as seen nowhere else on earth.

A refuge from ringing cellphones? Not so much anymore.

In the popularity contest between Yellowstone's natural wonders and on-demand phone service, park administrators appear to have lost ground on a 2009 pledge to minimise cellphone access in backcountry areas.

Signal coverage maps for two of Yellowstone's five cellphone towers show calls can now be received in large swaths of the park's interior including the picturesque Lamar Valley.

The maps were obtained by a Washington, DC-based advocacy group, Public Employees for Environmental Responsibility.

Yellowstone technology chief Bret De Young says officials are working with phone companies to install new antennas that should limit phone signals in the backcountry.

AP