Residential and business telephone users confused over whether to subscribe to a long-distance company other than the familiar American Telephone & Telegraph Co. now have access to a new cost-comparison service.

The service, offered by the nonprofit consumer group Washington Consumers' Checkbook, can cost from $10 to $600, depending on whether a residential or business phone bill is being evaluated and how many calls are made on a monthly basis, said Robert Krughoff, the organization's president.

If consumers send Checkbook a few months' worth of typical long-distance phone bills, the Checkbook computer will analyze what the bills would have totaled using different long-distance plans. The organization's computer is capable of evaluating more than 30 long-distance plans, including AT&T, GTE/Sprint, ITT, MCI Communications Corp., Satellite Business Systems and Western Union.

After evaluating 200 residential and business bills, Consumers' Checkbook found that the average cost difference between the lowest-priced long-distance company and AT&T was 29 percent.

But savings depend on calling patterns. In some cases, the difference between AT&T and the lowest-cost company was 7 percent, while for other callers savings were as much as 50 percent.

"There is no sure winner," Krughoff said. "It all depends on where, when, how long, and how much the consumer or business calls." The computer program developed by Checkbook takes into account distance, time-of-day calling discounts, volume discounts for large amounts of calling, charges for completing calls in areas not served by a company's long-distance network, rates for long-distance and "short" long-distance calling, the manner in which call-duration is timed and other factors.

"No one would want to figure this out on his or her own," Krughoff said.

The analysis does not include credit-card calls, operator-assisted calls, calls outside the continental United States, or calls within a local phone company's service area.

Reports are returned with tables comparing the features of each plan. Some of the companies listed in the tables may not serve all residential area codes. There also may be a local phone company charge to gain access to some long-distance services, Krughoff said.

Residential tables show whether long-distance plans have flat monthly fees, minimum monthly usage charges, higher rates in the first minute or higher rates to off-network locations. They include details on rate periods and volume discounts. Tables also show whether customers pay an initial subscription fee, how companies round off calling minutes for billing purposes, which locations customers can call and other features.

Business customers are advised of the quality of various long-distance services. Reports to business customers also include information about a number of business-oriented features such as travel-call procedures, autodialers, speed numbers, accounting codes and authorization codes.

In addition, tables in the reports to businesses include information on flat monthly fees, minimum monthly usage charges, higher rates in the first minute, rates varying with distance, rate periods, volume discounts, higher rates to off-network locations, minute rounding, and telecommunications management features.