Secure Science researchers were able to access accounts they had set up using an online service called , which allows users to make it appear as though they are calling from any number they wish.

Spoofcard has been used in the past to access voicemail accounts. Most famously, it was blamed when actress Lindsay Lohan's BlackBerry account was hacked three years ago and then used to send inappropriate messages.

The attacks on Google Voice and Skype use different techniques, but essentially they both work because neither service requires a password to access its voicemail system.

For the to work, the victim would have to be tricked into visiting a malicious Web site within 30 minutes of being logged into Skype. In the (pdf), the hacker would first need to know the victim's phone number, but Secure Science has devised a way to figure this out using Google Voice's Short Message Service (SMS).

Google patched the bugs that enabled Secure Science's attack last week and has now added a password requirement to its voicemail system, the company said in a statement. "We have been working in coordination with Secure Science to address the issues they raised with Google Voice, and we have already made several improvements to our systems," the company said. "We have not received any reports of any accounts being accessed in the manner described in the report, and such access would require a number of conditions to be met simultaneously."