iCall for the iPhone

1. Make and receive calls over WiFi
2. Transfer inbound calls from a regular cell call to WiFi instantly and seamlessly - save your expensive minutes
3. Access your same address book
4. Customize your voicemail options

Here is an example video

An actual screenshot of the iCall application running on the iPhone.
You can seamlessly switch calls from the cell network to VoIP.
Source: http://icall.com/iphone/