You can make free calls if you have a Gmail account. You don't have one? You can make it here.
Here is how to call using Google Voice:
1. Log in using your Google Talk account.
2. Please make sure that you have the "Advanced mode" activated. Go to YateClient Main Menu->Yate->Advanced mode, it should be checked, like in the screen below:
3. Go to Telephony tab in YateClient:
4. Set Protocol to none and Account to jabber:firstname.lastname@example.org. Type the number you want to call and then press the button.
Screenshots are from YateClient version 4.0.