As new cell towers continue to crop up around the state, access to cellular services appears to be improving for Mainers living in areas that long have lingered as wireless “dead zones.”

But as anyone who travels around the state can attest, Maine still has large areas where cell reception is spotty or nonexistent. And forecasting which areas of Maine soon will benefit from stronger cell signals is difficult, even for those who closely monitor the industry, given the highly competitive cellular industry’s secrecy.

“There has been significant and continual expansion of coverage,” said Wayne Jortner, senior counsel with the Maine Public Advocate’s Office, who handles telecommunications issues. “But there are still plenty of places where the coverage still is inadequate.”

Cell service providers such as U.S. Cellular, Verizon and AT&T have spent tens of millions of dollars — some of it federal funds — upgrading and expanding their networks in recent years. And at least one company, U.S. Cellular, already is offering the next generation of high-speed wireless service, known as 4G, in more populated areas of the state.

Determining the exact number of cell towers already standing in Maine — much less those in the works — is difficult, however, because there is no central agency that regulates towers or keeps tracks of those figures.

Permitting often is handled by individual towns although the Maine Department of Environmental Protection and the Land Use Regulation Commission process some proposals. Cellular service providers also often decline to release exact figures or future plans, citing concerns over the competition. And many towers are built and owned by a myriad of smaller telecommunications infrastructure companies operating in the state which then lease space to service providers.

AT&T spokeswoman Kate MacKinnon said the company has spent roughly $60 million in Maine on 400 “wireless system upgrades” between 2009 and 2011. U.S. Cellular has “more than 350 towers” in the state and has added 20 to 30 towers to its network each year for the past five years, according to spokeswoman Kelly Cioe.

Verizon Wireless spokesman Michael Murphy declined to provide tower figures, saying the number of towers in a particular area is not relevant when comparing coverage because low-frequency systems require less infrastructure than high-frequency systems. But Murphy said the company spent $297 million in New England in 2011.

One area where reception has improved is Hancock County’s Blue Hill peninsula, where for years “dead zones” seemed to outnumber the places where cellphones could pick up a basic signal, much less the more advanced networks used by most smartphones that provide Internet access.

At least three cell towers in Deer Isle and Brooksville have come online over the past year and more are planned, including one in Sedgwick that is under construction.

But reception problems persist in some areas popular with tourists, including downtown Stonington.

Town Manager Kathleen Billings-Pezaris said there has been a “vast improvement” in areas of Stonington that sit on higher ground since a U.S. Cellular tower in Sunset came online earlier this winter. Unfortunately most of the shops, restaurants, inns and other businesses are located on the wrong side of the hill.

“The only unfortunate thing is that right down here where we drop down to the ocean it is still very spotty,” said Billings-Pezaris, who is looking into options for “boosters” or other technology that would strengthen the signal downtown.

Kelly Kolysher, innkeeper at Inn on the Harbor in downtown Stonington, laughed that she sometimes can get a weak signal when the wind is blowing in the right direction. But she typically points people up the hill when they need to use their cellphones.

She and the owner are very upfront about the lack of cell service at the inn proper. While an inconvenience, the lack of reception is not a major problem and is regarded as a benefit by some visitors looking to truly get away, she said.

“More than you would imagine,” Kolysher said.

One obvious answer to the reception problem would be to locate a tower closer to downtown Stonington — a proposal almost guaranteed to encounter local opposition.

Cell tower siting can be tricky, especially in areas where 200-foot-tall metallic, latticed towers may mar the scenery that draws tourists. But those same visitors as well as local residents often want — even expect — to be able to use their cellphones.

That challenge is perhaps most acutely felt on Mount Desert Island, where Acadia National Park officials are exploring ways to expand cell service without detracting from the park’s natural beauty.

John Kelly, park planner at Acadia, said the intention is not to extend service throughout the park but to target key high-traffic areas such as Jordan Pond, Sand Beach and the top of Cadillac Mountain. In addition to enhancing emergency communications, advanced cell service could enhance visitors’ park experience by allowing those with smartphones to access special sites or apps about Acadia, Kelly said.

Kelly stressed that any discussions are still preliminary and that park officials will explore ways to expand cell coverage in the least disruptive ways possible, including by mounting antennas on existing buildings or using towers that are designed to blend into natural settings.

“We would not be putting cell towers everywhere or jeopardizing scenic views or resources,” Kelly said. “We are still in the very early stages of this plan and there are no commitments with any providers at this point.”

MacKinnon, spokeswoman for AT&T, said the company has identified eight potential sites both within the park or adjacent to it, although she said none are planned for this year. AT&T and any provider would need to work with the park service and therefore within federal processes to obtain authorization to build within the park.

“Expanding our wireless network to our customers who vacation in the park and who live and work in the area is something we are interested in pursuing, and we’re exploring all options to make this a reality,” MacKinnon said.

Verizon Wireless recently activated two new cell sites in the Bar Harbor area to increase coverage and capacity, according to a company spokesman.

The latest data from the U.S. Census showed that Maine was the nation’s most rural state in 2010, with more than 61 percent of the population living in areas with populations of less than 2,500. The state’s size and terrain present additional logistical and financial challenges for cell companies, although those challenges are by no means unique to Maine.

MacKinnon said her company and others face the same challenges in upstate New York, Vermont and New Hampshire. In the case of New York, AT&T has worked with state officials to expand the company’s infrastructure within Adirondack Park, an area that encompasses approximately 6 million acres of state and private land, half of which must stay “forever wild.”

“But this is what we do,” MacKinnon said of expanding in rural areas.

Cell service providers often receive substantial help from the federal government when building infrastructure in rural areas. That assistance comes from the Universal Service Fund, which is an assessment on telecommunications providers used to help pay for telecommunications services in rural areas.

In Maine, U.S. Cellular is the sole cell service provider participating in the Universal Service Fund, receiving $9.4 million for infrastructure projects in 2010 alone, according to state reports.

Cioe, the U.S. Cellular spokeswoman, said the company has tapped into roughly $8 million per year from the fund since 2007 to build and maintain 56 wireless cell sites in rural Maine. The company now has more than 350 cell towers or infrastructure sites across the state.

Cioe said the majority of those towns that benefited from the projects financed through the fund had populations of less than 5,000.

“Without USF for wireless, it would have been impossible to support infrastructure investment in these areas,” Cioe said in an email.

Of course, cell companies also are expanding services aggressively in Maine’s more populated areas.

U.S. Cellular recently launched the company’s 4G service in Portland, Bangor, Lewiston-Auburn, Ellsworth and the Presque Isle-Houlton area. 4G refers to the latest, ultrabroadband service capable of offering users with 4G-compatible devices much faster download and upload speeds than 3G service. Verizon and AT&T offer 4G service elsewhere around New England and plan to expand the higher-level service into Maine.

Darren Colvin, director of network operations for U.S. Cellular’s northeastern region, said upgrading to a 4G system requires new antennas that typically are added to existing towers. So while 4G is an entirely new system, it does not require companies to build new towers or other infrastructure, Colvin said.