Its possible to use the iPhone's or iPod Touch's internet connectivity to make low cost or free VOIP phone calls.
The iPhone can make Internet connections either via the cellular data network EDGE plus UMTS for the 3G version or via WiFi, while the iPod Touch can only use WiFi.
Using VOIP over the cellular network may violate your cellular provider's terms of service — check your contract first.

News

• 2009-05-15 - Taridium adds Apple iPhone interface to Enterprise PBX
• 2008-10-10 - Cost of VOIP on iPod Touch vs iPhone
• 2008-10-06 - Review: fring VoIP client for iPhone is no cell replacement
• 2008-10-06 - First iPhone VOIP Application in iTunes App Store
• 2008-09-11 - German court bans VoIP on iPhone by request of German provider T-Mobile.

FAQ

• Does Apple allow VOIP on the iPhone? — Steve Jobs says yes in a Q&A after a keynote speech
• Does AT&T (the iPhone carrier in the USA) allow VOIP over EDGE? — probably not?

VOIP Native Applications for iPhone

• iCall - product web link
• nimbuzz
• truphone - product web link
• Packet8 Mobiletalk
• Siax SIP and IAX client for iPhone (Commercial)
• Skype
• Fring iphone sofware - Free in iTunes App Store
• PhoneGnome - product web link
• sipgate software — only works with iPhone firmware version 1.1.4.

VOIP Web Applications for iPhone

• iPhone interface for Taridium ipbx (integral part of ipbx and ipbx eXpress)
• iphone Web application from Jajah
• Phonegnome
• Talkety
• Raketu
• RF Dialer web based dialer - works with most VoIP providers, Asterisk, and Skype from RF.com

Resources

• iPod touch VoIP Apps - The Best Free Phone Call Apps & Services

See Also

• SIPHON SIP for Apple iPhone and iPod Touch
• Global IP Solutions GIPS VoiceEngine™ Mobile supports the iPhone
• Wireless VOIP