It's free for the person initiating the call on their computer. Cell phone companies never, as far as I know, extended free minutes to incoming calls as is the case for land lines. So, obviously any minutes on a cell phone would be charged any fees regardless.

The per-minute charge for cellphone minutes is intended to cover the cost of delivering the call over a radio channel, which is an expensive and limited resource. European cellular carriers will often charge the caller the full cost of the call (using a special area code which has a higher per-minute rate) and allow customers to receive calls without using up their own money.

This was tried in the United States with area codes 500 and 533. It didn't take off, probably because callers didn't like the cost of a call being unpredictable (carriers could set a different call cost for each 500-NXX block).

in Brazil any number starting with 7,8,9 are cell phones. you know you will be paying cellphone call price. Just like it is with VoIP.

In California i waste most of my minutes receiving "usted ganhaste la loteria" calls.

how exactly is it free then?

person X sitting on their computer, they click call person Y.

google will first call person X's mobile (he will pay to receive) and then call person Y's mobile (he will also pay to receive) and connects both.

so, where is the free part?

And since the monetary currency in mobile bills are minutes, you are paying the exact same thing as if person X just picked the phone and called person Y.

actually, if he did that, then it may be really free as operators gives more minutes for mobile to mobile calls! which is not the case when receiving a google voice callback by starting the call in the browser.

The new Google phone service is entirely on the PC. It doesn't call your phone like the original Google Voice.