SIPphone, the software company behind the Gizmo Project, is offering free internet phone calls to landlines in 60 countries, and mobile phones in 17.

There are restrictions, of course. Callers and those who receive calls must be registered and use the Gizmo VoIP (voice over IP) software on a regular basis. The company says on its website that it "reserves the right to limit call length".

SIPphone says the program means access to more than two billion landlines and mobiles.

Gizmo Project appears to be one-upping competitor Skype, which announced in May that calls originating in the US and Canada to landlines in those two areas would be free until the end of the year. Calls outside of those areas are billed at Skype's rates.

Gizmo offers other VoIP features similar to Skype, including free PC-to-PC calls and rental of a US phone number that rings your computer. The software's IM (instant messaging) service is compatible with Google's Talk IM application.

Eventually, Gizmo and Google Talk users will be able to call each other, according to Gizmo's website.

The Gizmo Project software can make calls to landlines of users not registered with them, which are billed at varying rates according to the country dialed. Full details are here.

Earlier this month, Nokia said it would include a special Gizmo software client on its 770 internet tablet. The latest version of the 770 has Wi-Fi and runs a Linux OS.