You’ll need a Gmail account; get it here: http://gmail.com if you don’t have one yet.
Then go here: http://voice.google.com to get a Google Voice account (login using your Gmail account).
Choose a number for your Google Voice
(If you need to get a number local to you, stop here and come back later if you don’t see a number in your area code or zip code. Google maybe out of numbers in your area but they add numbers often; you just have to check back often if you don’t find numbers in your zip/area code).
Once you are done selecting a number, go to Options (gear icon), Voice settings
In the “Phones” tab, add a phone (to forward your Google Voice calls to)
Google will call the phone you are adding to verify the phone; you'll need to input a 2 digit pin displayed on the screen for the verification.
Click on Edit (the phone you added), click on “Show advanced settings”
If you want to have PIN less access to your Google Voicemail, click on “PIN not required” radio button under "Yes, direct access to voicemail when calling your Google number from this phone?"
Once you've added a phone like above, when you call your Google Voice number, you'll be greeted with "you have no new messages" and then "to place a call, press 2"
To place calls using your Google Voice number, once in your Google Voice voicemail, press 2 and then the 10 digit number + # for US and Canada (free calls); 011 + country code + number + # for international calls (need to load $ into Google Voice account but initial balance is 10 cents).