– At Yosemite National Park, hikers are likely to encounter people talking on cellphones as they climb to the top. For visitors to the parks, the call of the outdoors increasingly comes with crisp 4G service, and not everyone is wild about that.

In Yosemite, Yellowstone, Glacier, Mount Rainier and other iconic parks, environmentalists are pressing the National Park Service to slow or halt construction of new cellular towers within park boundaries. They say the NPS is quietly facilitating a digital transformation with little public input or regard to its mission statement — to preserve “unimpaired the natural and cultural resources and values of the National Park System.”

Richard Louv, author of several books on connecting young people with the outdoors, said the parks are losing what once made them unique.

“Can you imagine hiking in Yosemite far from other people, and then suddenly it sounds like you are in McDonald’s, with everyone on their phones?” he asked.

Yet advocates for increased cell service, including many NPS officials, say the parks can’t cling to an earlier era. Expanded cellular and broadband coverage, they argue, helps rescue teams respond to emergencies.

“Visitors want to be able to use their mobile devices to share experiences with their friends and family,” Lena McDowall, an NPS deputy director, told a U.S. Senate subcommittee in September. “They want to take advantage of the many internet-based resources we have developed.”

Verizon, AT&T and other telecom companies are aggressively courting the most popular national parks, and under the federal Telecommunications Act of 1996, the parks are obligated to at least review proposals for new cellular towers. Because the NPS is highly decentralized, its headquarters does not track construction of cell towers in parks, nor has it developed a policy to guide reviews of the proposals.

Yosemite is one park that has come under scrutiny for its expansion of cell service. In October, using public records request, the watchdog group Public Employees for Environmental Responsibility found that Yosemite has quietly approved six cellular towers.

PEER, which has asked the Interior Department’s Inspector General to investigate, said that Yosemite is in “violation of both federal laws and agency policies” by approving the towers without public notice or environmental review.

Juggling public demands has always been difficult in the national parks, especially those that draw big crowds and include large expanses of designated wilderness. In 2016, the NPS reported a record 331 million visits to the parks, many of which suffer from overcrowding in the summer.

For the last year, Mount Rainier National Park in Washington state has been weighing whether to allow three telecom companies to co-locate a cellular facility at the park.

Public opinion appears divided. Of those who commented, 249 people were supportive and 241 were opposed.

In North Dakota, wilderness advocates strongly opposed Verizon’s plan to build a new cell tower at Theodore Roosevelt National Park, fearing it would blanket the backcountry with cell signals. NPS officials ultimately decided to design the new cell tower so it would not extend service into the park’s wilderness.

Under National Park Service guidelines, “special uses” should be rejected if they “unreasonably disrupt the atmosphere of peace and tranquillity of wilderness.”

U.S. Rep. Jared Huffman, D-Calif., introduced legislation that would allow parks and federal land agencies to keep the rental income they receive from granting right of way to cellular towers.

“If you want to avoid distractions in the wilderness, you can just turn off your phone,” he said. “But you might also want to be able to turn on that phone and make a call if you broke your arm.”