If long-distance phone bills in your home run $10 to $25 or more a month, it may be worth your while to check into one of the communications companies offering long-distance calling at rates lower than those charged by AT&T.

The four companies on the scene so far are: International Telephone and Telegraph Corp. (ITT), MCI Telecommunications Corp., Southern Pacific Communications Co. (SPC), and Western Union Corp.

Which company offers the best rates for you depends on where and when you call most often, plus other factors, but savings can be significant -- particularly during evening hours.

These "specialized common carriers" claim you'll save 30 percent on weekday daytime long-distance calls, 50 percent on weekday evening calls (before 11 p.m.), and 15 percent on calls placed after 11 p.m. on Bell-defined weekends (11 p.m. Friday up to 5 p.m. Sunday).

AT&T points out that the reason these firms can charge slightly less is that they're "skimming the cream" off AT&T's long-distance business by serving only the high-traffic areas. "We have an obligation to all of our customers, not just the big ones," says Washington spokesman Mike Houghton. "Our toll rates are based on nationwide averages. Our competitors have a lot of volume between the high-traffic areas. It helps them, but it won't help the callers in Oshkosh."

Although residential customers can get 24-hour service from these companies, special rates are available for subscribers willing to limit their service to nonbusiness hours, when their facilities are underused. Three of the firms levy a $10-a-month service charge for full service, but charge only $5 a month for evenings and weekends only. The fourth, Western Union, has a $40 monthly minimum for 24-hour service, $10 for off-peak (effective March 15). But variations in rates and plans are seemingly endless.

One useful summary of the wide array of rate variations is contained in this month's issue of Consumer Reports, along with a synopsis of its experiences in logging 160 long-distance calls on the four services. One of the chief problems Consumers Union experienced was with transmission.

"Our connections were hardly ever as loud and as clear as we've come to expect on a Bell System long-distance call," says the report, "though poor connections on its lines are not unknown." (SPC, according to Consumers Union, had the best transmission.)

Despite these problems, MCI, the oldest and largest of these firms, claims to serve 300,000 customers (80 percent residential).

Because you can make long-distance calls from any push-tone phone near an access line, these services may also function as a convenient credit card for travelers, or for members of the family who regularly call home collect -- and you don't incur a charge for operator assistance.

MCI charges $5 and Western Union $2 a month for away-from-home service, the others nothing. Each firm has its own set of charges, however, so it's advisable to study all brochures.

Some other example of rate variations: ITT charges an initial $10 and SPC $15 to set up residential service. Some firms charge by the half-minute; MCI and Bell by the minute (which means that if you talk for 61 seconds, you'll be billed for two minutes). SPC charges 10 to 15 cents a call.

For high-volume users, ITT offers a comparatively economical plan: a $10 start-up charge, and $2.50 an hour for up to 10 hours of phone time (with a 2-hour minimum).

Some companies serve far more areas than others, but you can figure out which will serve you best by comparing your recent calling patterns with their specific offerings. The plans are sometimes quirky; only Western Union serves Seattle, for example, and only ITT serves Florida. Some firms allow intrastate calls; others do not. (So far, none of them has overseas service.)

For further details, contact:

ITT, City-Call, (202) 293-1470.

MCI, (301) 251-8900.

SPC Sprint Service, (703) 527-4929.

Western Union Metro I, (202) 624-0351.

For businesses and larger-volume callers, there are even more options, including Bell's WATS line and the so-called network management system. But that's another story.