3 UK today announced a free SIM that lets anyone make Skype-to-Skype calls for free from any unlocked mobile phone.

From a compatible, unlocked 3G handset anyone can use Skype via the 3 SIM, whether or not your phone is from 3, without ever having to pay a penny.

The deal also covers Skype instant messages, and free chats on Windows Live Messenger.

Is it the magic number?

To get the deal, you have to sign up on 3's website, check that your 3G handset is compatible, apply for a SIM, fit it and install 3's Skype client. Of course, the service will only work on unlocked phones.

"The expansion of our relationship with 3 indicates another leap forward in providing people across the UK with free internet calling on their mobiles," said Russ Shaw, European General Manager. "The demand for mobile access from people who use Skype has never been higher and Skype, in partnership with 3, is committed to developing new ways to provide people with an enhanced experience."

3 UK already has around 350,000 Skype users, using an average of just under an hour's free Skype-to-Skype calls every day. The deal works for 3 because the network has found that regular Skype users are more loyal to 3 and use more traditional voice minutes and send more texts than other customers.