International Calling : Google Voice :

Make international calls

You can make international calls at very low rates with Google Voice. You can use the Google Voice website or the phone system, as long as you have one of your forwarding phones or your mobile device available to complete the call.

Get started by purchasing credit through your Google Account on payments.google.com. Rates vary by country and will be counted on a per minute basis. To view international calling rates, click the View all Rates link in the Billing tab of your Google Voice Settings page.

You can make calls outside the United States from the Google Voice website or through the Google Voice phone system:

• From the website: Click the Call button at the top left of the page and enter "+country code" or "+1countrycode" (depending where you're calling) followed by the international number. For example: +442012345612345 or +1246-426-0200.
• From the Google Voice phone system: Access the Google Voice phone system by either calling your Google number, or if you're using Google Voice Lite, calling your access number from a mobile phone registered on your account. Once you're in the Google Voice system, press 2 to make a call. When dialing an international number, enter 011, the country code, and then the number (for example: 011442012345612345).

Via : https://support.google.com/voice/answer/141922 & https://www.youtube.com/watch?v=y6Zy-Ande6I