The Federal Communications Commission has agreed to look into allegations by Vonage that an incumbent service provider deliberately blocked Internet telephony calls made over its broadband network.

Vonage has made informal complaints about the problem, which it says affected as many as 200 voice-over-IP customers in late 2004. The VoIP provider has not publicly identified the service provider involved.

Vonage CEO Jeffrey Citron told The Washington Post that Vonage engineers visited customers’ homes and discovered that the local telephone company had blocked the Vonage data stream, preventing its customers from making VoIP calls. Citron said Vonage was able to route traffic around the blocks to enable customers to keep calling.

The Vonage complaints could become the basis for a broader inquiry into the “gatekeeper” role that broadband service providers such as telcos and cable companies could play and whether they have the right to discriminate against competitors, or provide favorable treatment to partners and sister companies, while controlling access to the Internet. Content providers have raised concerns in the past that companies that control access can use that control to their commercial benefit by blocking or delaying access to specific data packets.