Under pressure from the F.C.C., Cherry Communications last year agreed to pay $500,000 to the Federal Treasury. The payment was described by the commission as "a voluntary, non-tax-deductible contribution."

But AT&T, MCI and Sprint have also been accused by some consumers of slamming. An AT&T spokesman in Queens, Richard G. Gundlach, said some complaints may involve one member of a household accepting an offer to change service but not telling the person who opens the phone bill; other complaints may be the fault of clerical error, he said, since big companies process thousands of requests a month.

The size of the problem is not clear, but consumer groups contend that it is very large. Mr. Gundlach said that in a six-month period last year, approximately 5.6 million of AT&T's customers, 7 percent of them, were slammed.

Among customers whose primary language was Spanish the rate was 18 percent, he said. According to regulators, some companies target immigrants from Latin America or Asia, using "contest entries" mostly in Spanish or Chinese, with the fine print in English.

Regulatory agencies catch only a sample of the problem, but they do document a growing trend. The New York Public Service Commission received 281 complaints of slamming last year, including a handful against Sonic. So far this year it has received 80 against Sonic alone, said Edward Collins, a spokesman. In Washington, the F.C.C. reported 2,500 complaints last year, up from 1,700 the year before.

Slamming, a term whose origin is not clear, represents a new front in the telephone wars. For years, credit-card holders using a pay telephone have had to be wary of the possibility that the service would be provided by an obscure company that charges double or triple what AT&T, MCI or Sprint would charge; now, the hazard extends to the family telephone as well.

In theory, consumers choose the long-distance carrier they will use, and must authorize changes. The F.C.C. calls the authorization letter a "letter of agency." But some small phone companies have been hiding letters of agency in sweepstakes entries for a car or trips, with fine print saying that by entering a name and phone number, the consumer authorizes a change in telephone service.