In an action that could increase telephone bills in rural areas but lower rates for some long distance calls, the National Telecommunications and Information Agency has proposed sweeping changes in the methods used to compute phone bills.

The agency, part of the Commerce Department and the focal point for administration communications policy, is attempting to moderate the growing battle over how to deregulate fairly the telephone industry, which for years has been dominated by the Bell System under a government-sanctioned monopoly.

But with deregulation gradually allowing new forms of competition for American Telephone & Telegraph, the administration is seeking a way to allocate the cost of communications services fairly.

Because Bell is still the cornerstone of the nation's communications system, the government oversees a set of complex rules that set the rates competitors must pay to route their business over local Bell lines. Several companies, for example, offer special long distance telephone services, but in most cases a portion of the actual call must travel over Bell-owned lines.

AT&T has been concerned that it is not being reimbursed adequately for providing that service to its competitors.

The NTIA proposal calls for a two-phase plan that would alter dramatically the formulas used to compute those charges over the next 15 years, while at the same time take into consideration the interests of the various segments of industry involved.

When the costs of making a telephone call are realocated, however, industry sources say it is likely to upset sharply the present method of customer billing and lead to major changes in the fees for various services.

Essentially, the proposal would make each type of service, or each long distance call, pay for itself. AT&T has long claimed that it has had to charge more for long distance calls because it is subsidizing local service, which is kept at a low rate by state regulatory agencies.

Under this proposal, cost effectiveness would place a heavier burden on less populated areas. Service to a single residence 30 miles out of town costs the phone company more to provide than service to a single unit of a high-rise apartment complex, for instance.

The plan, which NTIA says is still a "working plan" and not yet endorsed by the administration, calls for a preparation period of two years during which state regulators, who have jurisdiction over telephone rates, would work with a new federal "transition board" that would plan for the revisions by studying all available data on telephone costs.

The transition board would seek a method of allocating costs, and seeing that companies pay each other adequate compensation for shared services.