You can currently call landlines and cell phones (mobiles) using Google Voice, from within your GMail account. As long as you reside in the US or in Canada.
It appears that Google is about to extend this feature to the rest of the world. The following are screenshots taking this morning which show that the functionality is gradually being extended to the rest of the world.
First of all, log in to your GMail account.
If it works for you, you are greeted with the above hint about making calls. By clicking on Try it now, you activate the new feature.
You need to accept the above. The rates are quite competitive, and there is comparison with some other VOIP provider, most probably Skype. Google Voice does not have a connection fee for the calls (unlike Skype, which does have a connection fee).
Then, you install an addon so that you can make phone calls. See http://www.google.com/intl/en/chat/voice/ I have been testing this on Ubuntu 11.04 (64-bit) and I installed the appropriate package.
Once you activate the service, you now have the option to Call phone from the Instant Messenger (IM) app of your GMail.
At the moment I get the message that I cannot add credit to my account, so I am lucky enough to have some left-over credit from I-do-not-remember-from-where.
Now we are making a phone call to a landline in Greece. It shows the cost, $0.2/min.
My credit in this account is in US Dollars, and somehow I cannot top up using Google Checkout because “Google Voice is not available in your country”. However, in an other account, with no existing credit, I get the option to top up with Google Checkout, in British pounds.
There are reports that users can get a +email@example.com SIP address. I believe that eventually users will be able to access their Google Voice accounts using SIP, so that they can use their preferred VOIP client software. At the moment it may or may not work for you. My attempts to call using Empathy, Jitsi, SFLPhone were currently unsuccessful.
Having the ability to call landlines and mobiles from Google Voice (within GMail) is big. Google did not market yet well enough the Google Voice PC to PC features in GMail. Now, with Google Voice to landlines and mobiles, Google has a complete offering for VOIP that can attack Skype at their core business. The VOIP business is changing rapidly.
Here is the Google announcement for Google Voice all over the world, http://googlevoiceblog.blogspot.com/2011/08/calling-from-gmail-now-in-38-languages.html