New iPhone SDK 3.2 Now Allows VoIP Calls Over 3G
http://ping.fm/PPXT3

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There are two ways to phone through IP-phone. You can phone from the web-site with the help of your personal account and with using special program installed on your PC. Also, during IP call you may use any additional headsets if you do not have built-in loudspeaker. So, with using headset and microphone you will hear your interlocutor.
http://www.voip-catalog.com/ Voip providers