As a promotion, Skype began allowing its users to place free domestic “SkypeOut” calls from their computers to traditional and mobile phones last May. At the time, the company said the promotion would extend only through year’s end. The company is offering a half-price subscription to those who sign up before Jan. 31. Calls from one computer to another have been and will continue to be free.

“We see a willingness by consumers to make SkypeOut calls that are well priced,” said Don Albert, Skype’s general manager for North America. He noted that the cost was still a fraction of the typical $25 monthly fee that other Internet phone providers charge for unlimited calls. Mr. Albert declined to predict the adoption rate for the plan or the revenue it could bring in.

Despite the relatively low cost of the service, industry analysts said Skype was not considered to be serious competition in the telecommunications business. Skype, unlike Vonage, the cable companies and other competitors, generally requires users to download software and to make calls from the device on which it is installed.

“Skype requires a behavioral change. Consumers have grown quite comfortable using their telephones,” said Jeffrey Halpern, a telecommunications services industry analyst with Sanford C. Bernstein & Company. “I don’t view Skype as a real threat to the telephone companies or even Vonage or the cable companies.”

Over all, the Internet calling business is booming. Mr. Halpern said that by the end of the third quarter, there were around 8 million subscribers to Internet calling plans in the United States, up from 6.5 million in the previous quarter. That figure did not include users of Skype.