Hackers are targeting VoIP users in a bid to steal usernames and passwords so they can make internet telephony calls for free.
According to VoIP equipment manufacturer Newport Networks, the usernames and passwords of VoIP accounts are being sold online by hackers for £9 each. In contrast, stolen credit card details are traded by criminals for £6.
Dave Gladwin, vice president of products at Newport Networks, said that while customer details are encoded when sent across the network, the data can be "easily captured and unobscured".
"It is still at an embryonic stage but as voice adoption increases it becomes more of a problem and needs addressing," Gladwin told BBC News.
"90 percent of carriers don't offer a secure VoIP service," he added.
"Most of the software out there has the capability of running in secure mode if the service providers would accept it," he said.
However VoIP provider Skype insists it offers end-to-end encryption.
"It doesn't matter whether I'm on an open wireless connection, there is no way someone could get hold of my username or password," said Jonathan Christensen, general manager of audio and video at Skype.