Vonage Holdings Corp. said it is investigating new potential incident of its Voice over IP service being blocked, this time by a cable television company that also provides Internet services.

Brooke Schulz, Vonage's vice president for corporate communications, confirmed that the company is "investigating a new instance" of service interruption that appears to be another case of port blocking. Schulz said the incident involves Vonage customers who use high-speed Internet services provided by a cable operator, somewhere in the Midwest U.S.

Last week, the Federal Communications Commission reached a consent decree with Madison River Communications of Mebane, N.C., under which Madison River agreed to pay the government $15,000 for its action of trying to block Vonage's service, and to never block Vonage's VoIP service in the future.

Over the past two weeks, industry sources who declined to be named said they had heard rumors that some Vonage customers in the Midwest were having their services blocked. On Wednesday, Schulz confirmed that Vonage is "investigating an instance [of service outages] on a cable operator's system in the Midwest."

Vonage declined to name the operator. Schulz said that Vonage does not yet know the exact number of customers affected, but that it could range "from the tens to the hundreds to the thousands." Schulz said that Vonage was able to re-establish service for several customers who complained by rerouting its VoIP traffic.

Unlike the Madison River incident, Vonage has not voiced any complaint about the new incident to the FCC, Schulz said.

"We are still investigating, and expect to have more facts next week," Schulz said. "But we are taking this very seriously."