The problem, which surfaced about two weeks ago, apparently prevents some Net phone users who are also broadband customers of Adelphia Communications, Citizens Communications' Frontier and various rural cable providers from receiving calls, according to sources familiar with the situation.
A handful of AT&T's CallVantage Net phone subscribers are among those affected. "We are seeing, sporadically, some instances of blocking by some cable company networks," AT&T spokesman Gary Morgenstern said.
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AT&T plans to release a patch to fix the problem in the next few weeks. It will be part of a previously scheduled software upgrade for its customers, according to Morgenstern.
A Frontier representative said Wednesday that the company was not deliberately blocking Net phone calls and was unaware of any complaints.
A representative of Adelphia could not be immediately reached for comment.
Because of thefor selling these cheaper phone calls, some angry customers are suggesting that cable companies, which also sell Net phone services, are deliberately blocking the calls.
But others, including Morgenstern, believe that there's no foul intent; rather cable companies are trying to safeguard egresses to the Internet typically used by, he said.
Voice over Internet Protocol (VoIP) calls use a particular application network address, or port, to communicate call setup information. Ports are numbered from 1 to more than 65,000 and allow applications to listen on the network for data from other applications. In this case, the call setup information is carried on port 69, which is nonstandard for VoIP calls. Most VoIP calls normally use a different port number, 5060.
While broadband and backbone providers do not typically block specific ports, port 69 is also used by a simple file server, known as the trivial file transfer protocol, which is a component of the MSBlast worm. That port was blocked by many providers for a few weeksa year ago.
Morgenstern said the temporary measure may have been reinstituted by providers whose customers were hit by that virus or others.
"We don't think it's targeted at VoIP providers," he said. "Cable companies are doing it because they believe it's one of the ways to prevent some worms and viruses."
He said the impact has been limited to just a handful of customers ofNet phone service.
Net phone calling, which typically uses VoIP technology, is winning converts, thanks to cheap rates and a slew of features that traditional phone companies can't match.
There are now only about 600,000 commercial Net phone subscribers, a market too small for the attention of big telemarketers. But the. Technology research firm IDC predicted that VoIP revenue will grow from $3.3 billion in 2003 to $15.1 billion by 2007.