Packet8 happens to be my VOIP provider and they have offered discounted international rates from your cell phone for $4.99/month. Not really worth it. But temporarily they're letting people sign up for no monthly fee "for the life of your service!"
Packet8 VoIP International Calling Using a Mobile Phone
You download a little app to your phone which intercepts calls starting with 011 or a + and then routes it to the Packet8 network. Rates are pretty good and as far as I can tell are the same rates I get for my home phone. Billed to a Packet8 account you create and not your cell service provider. It says it's a virtual calling card.
If you store all your phone numbers with a + in front like I do, it will let it dial normally if you're calling from the same country you're calling to. So all my US calls are completely unaffected.
When I called an international number, it dialed an 858 area code (San Diego, CA) number followed by a ton of digits, then connected my call.