Skype for iPhone will enable you to make calls through a WiFi network. It's rumoured that the app will enable you to make Skype-based calls over your 3G network, using the forthcoming iPhone software revision 3.0. No doubt this will annoy the mobile providers, but as Skype is primarily used for long-distance calls, it shouldn't affect their revenue streams too greatly.
Of course, you can also use the Skype app to handle your messaging. We use Skype for keeping in contact with key software developers, for example. As this will work on the iPod Touch, you could connect a microphone with the revision 2 and use this as a phone.
Skype for iPhone link.
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