Vonage can’t seem to catch a break lately. The company has fought and lost a number of patent infringement lawsuits this year including two against telecoms Verizon and Sprint, and now AT&T is taking the VoIP firm to court too. On top of that, security firm Sipera Systems is now claiming that Vonage’s internet phone service is vulnerable to attacks by hackers, who are able to intercept calls to the company’s subscribers.

Sipera Systems said it had informed Vonage of the problem more than a month ago, but the company had not responded to the warning. According to the security firm, the Vonage VoIP Motorola Phone Adapter (VT 2142-VD) does not authenticate SIP requests, leaving users vulnerable to a form of VoIP identity theft that could allow a hacker to take over a user’s phone service.

Sipera says it also found problems in services offered by Globe7 and Grandstream. But considering that the company who released these warnings also happens to sell the equipment designed to protect VoIP customers from these forms of attacks, one has to wonder if this is just a case of product marketing.