A lot of people know that Skype has ability to make a free call with North America until the end of 2006. I tried before, the voice quality is good enough.
But I am tied to my computer. The skype is only run on the computer.
Voxlib is a solution. 2013.08.29 Update: This site/service is down.
1) Install a skype and create a new account.
2) Download Voxlib client software, Vox for Skype, and Install it.
3) Setup your cellphone information in Voxlib.
Go to try it.