Turn Your iPod Touch into an iPhone

[Butcho]

The iPhone and iPod touch are almost indistinguishable devices except for one major difference—you can make calls from your iPhone, and you can't from your iPod touch. For the privilege of making phone calls with your iPhone, you have to pay $100 more upfront to Apple for the device itself, plus a minimum of $60/month to AT&T for the next two years. Let's say you didn't need that kind of firepower from your iPod touch, but that you would like to use it make a phone call every now and then. You can, and today I'm going to show you how to make VoIP phone calls from your iPod touch or iPhone using a freeware application called SIP-VoIP....

Instructions

[danimal]

That's a pretty sweet tutorial, I should do it sometime.
The headsets are pretty expensive though.

[user_account]

WLAN-SIP-phone != cell phone

Not a bad hack, though.