SAN FRANCISCO - Cellular subscribers are paying hundreds of millions of dollars each year to subsidize landline telephone service, enriching big telecommunications companies while providing little or no benefit to cell-phone users.

The subsidies are intended to reimburse the companies for providing traditional phone service in rough terrain and rural areas where stringing lines can be costly. But rampant development has transformed some of those backwaters into booming subdivisions, with no real adjustment to the distribution formula; others, such as the oceanfront celebrity playground of Malibu, are receiving subsidies simply because of their difficult topography.

Outdated formulas for tabulating the surcharges - coupled with feeble government oversight - have meant a windfall for phone companies, which are fighting to preserve them.

"It's egregious," said Kimberly Kuo, executive director of MyWireless.org, a national nonprofit advocacy group for cellular users. "By nature, these fees are highly discriminatory because cell users pay in far more than they get out of it."

Nineteen states charge customers a fee to defray the costs to phone companies of providing service in high-cost areas. Of these, 12 do not exempt cell phones - Alaska, Arizona, Arkansas, California, Colorado, Kansas, Maine, Nebraska, Nevada, Texas, Utah and Wyoming.

Since 2003, those states have together collected more than $4 billion, an Associated Press investigation found. The burden is shared by cellular and landline phone customers alike. In some states, cell users appear to foot more than half the bill.

"There's an enormous inequity with wireless contributions," said Joe Farren of CTIA - The Wireless Association, a trade group representing the nation's cellular providers and wireless-equipment manufacturers. "We think these funds should be no larger than necessary and not favor one technology over another. It's a major issue for us."

Phone companies also pay into a separate federal Universal Service Fund that has raised nearly $20 billion since 2003. Some of that money subsidizes landline service in hard-to-reach areas of every state. The Federal Communications Commission does not require telecoms to pass those costs along to their customers, but many do. Cell-phone users pay into the federal fund, but it is difficult to determine how much of it they contribute.

Industry officials say the subsidies - known as high-cost universal service funds - are what make it worthwhile to do business in rural areas. If they were abolished, some other incentive would be needed.

"These are tough issues," said Phil Cleverly, director of regulatory affairs for Verizon California. If surcharge subsidies are not continued, policymakers "will have to decide how it is local rates in rural areas should be supported in the future," he said.

Most consumers overlook the surcharges on their telephone bills. Usually no more than a few dollars per month, the charges support a variety of programs, including those that ensure affordable telephone service for low-income and disabled customers. But the high-cost subsidies are the most expensive and possibly the least regulated.

In California, for example, the two biggest phone companies, AT&T Inc. and Verizon California, received $1.2 billion in subsidies over the last three years as compensation for serving more than 7,600 designated high-cost areas. That list has remained static for years and is based on the 1990 census.

The state's 25 million cellular subscribers contributed 60 percent of those payments. California's full universal service program includes five funds and has received $2.8 billion since 2003.

"We believe the core principle is that everyone, including cell-phone callers, benefits from being able to call people in the high-cost areas," AT&T spokesman Gordon Diamond said. "If wireless customers didn't contribute, the surcharge on wireline customers would have to be higher."

In most of the states, where the money goes and how it is divvied up among phone companies is often shrouded in secrecy because government-sanctioned reporting procedures cloak the information from public view.