This one's been kicking around for a while, but with CyanogenMod 7 nightlies getting better, and the hard work Unforgiven512 has done with the files, WiFi calling for the Nexus One is now worth using and sharing with you guys. To clear up any confusion, we're talking about routing calls through your existing home WiFi, and not VoIP calling. It's the same method as the MyTouch 4G, and other T-Mobile Android phones -- just ported over to the Nexus One.
WiFi calling on Android uses your voice minutes, so you're probably wondering, why do it? Besides the call quality (it's top notch, almost land-line quality) it's a helluva battery saver. It's also the perfect way to make calls outside the T-Mobile service area, without incurring any roaming costs. Speaking of T-Mobile, you must have an active T-Mobile SIM card inserted to use WiFi calling. Other carriers don't have the infrastructure in place to use it. Maybe one day they will, as it's a perfect solution for spotty coverage at home.
If you're interested, and rock a Nexus One with CyanogenMod 7, hit the Android Central forums for the guide to get started. Big thanks to Unforgiven512 for the hard work with this one!