Theoretically, if you used this with My3G (which enables WiFi-only apps to work on 3G), Line2 could really become a full-fledged phone replacement for the iPhone. You wouldn't even need to have a calling plan or texting plan, since Line2 can do it all for free. All you need it data.

Somebody should try that out. :D

My3G (jailbreak required)

Line2 is a decent little app that lets you listen to messages left on your Toktumi message system (see Toktumi listing on AlternativeTo 'Desktop' site)

Although you can place and receive calls via VoIP on your iPhone when you are connected to a WiFi network I have found that it works however the latency is worse than most VoIP systems unless you're on a WiFi network that has been optimized (QoS) for VoIP telecommunications.

Don't let that dissuade you from at least trying the app. It has saved me potentially thousands of dollars in calls, messages, and business.