But the Madison River case may not be an isolated incident. Citron said Vonage is investigating incidents involving other small telephone companies around the country.
Christopher Libertelli, Powell's senior legal advisor, said other VoIP providers have brought similar complaints to the commission, and they are being evaluated.
According to Nuvio Corp., a Missouri-based Internet phone provider, two cable companies that provide Internet access appeared to be degrading its service.
Jason P. Talley, Nuvio's chief executive, said that when his customers complained to their cable companies, they were told that if they switched to the cable company's voice service, the problems would disappear.
"That raised the hair on the back of our necks," said Talley, who declined to name the cable companies.
Unlike outright blocking, Talley said, degrading service by introducing delays or dropping occasional calls is difficult to prove.
Some consumer groups, and even a few major technology corporations, have called on the FCC to establish strong "network neutrality" guidelines.
They say this is especially important as the current FCC pushes to reduce rules governing Internet services provided over phone lines. Cable firms are free from regulations over their Internet services, but that is being challenged in a case that is scheduled to be heard by the U.S. Supreme Court on March 29.
Major cable and phone companies, while saying they have no intention of engaging in network discrimination, nonetheless oppose formal guidelines or rules.
Powell said yesterday that the proper way to handle the issue is through case-by-case enforcement.