I found this to be pretty handy because this solution allows you to call any cellphone/landline number in the US for free. I might as well share it:

Things you will need:

  • Something that will result in you having an american IP (hotspotshield, proxies, ... I used a server in the US and did SSH Forwarding)
  • a SIP account (you can get them basically everywhere. ekiga probably works. I used my dus.net account.
  • a service that will forward an american phone number to your SIP account. A good service for that is ipkall
  • last but not least: a google voice invite <- don't have any atm, sorry

How to do it:

  1. First, you've got to set up ipkall to forward the american phone number to your sip account
  2. Then: Click then invite link with your proxy/forwarding/... active so google will detect a US IP
  3. In the signup process, google will ask for a US phone number, just take the one ipkall gave you
  4. Google will then show you a number and call the american phone number. You have to type in the number you see to verify that you actually are in possession of that number. You sometimes have to check how DTFM signals ("the beeping") are actually handled (in-band, out-of-band). As far as I remember, I used "in-band", the lamest yet easiest solution
  5. After that you should have a google voice account that you can use to call the US for free

After you have an active google voice account, all you have to do is tell google what number to dial. You don't need a proxy for that anymore.
Google will then call your "american landline" first, ipkall will forward this to your SIP account. Upon picking up the phone, google will start dialing the number you set on the website and from then on, it'll be just as if you dialed that number yourself. Just in this case: it's free.
I'm currently also using google voice to make calls to Germany from the US for 2 cents/minute. In that case, I could actually put in my US cellphone number though :)