If you have a smartphone, you no longer need to pay for data AND voice minutes. This video will show you how to make calls for free using Google Voice.
http://voice.google.com
http://www.sipgate.com
http://www.voxox.com
http://www.ipkall.com
http://www.appbrain.com/app/sipdroid/org.sipdroid.sipua
http://www.cyrket.com/p/android/com.xinlu.gvdial/
Instructions for iPhone:
http://www.hutsby.net/2010/02/howto-iphone-sip-isipsimple-and-sipgate.html
Proxy Instructions: