The Nexus One may be growing long in the tooth, but it's still surely one of the most active phones when it comes to development. Hence this hack should come as a surprise to no-one: T-Mobile's WiFi-Calling functionality has been extracted from one Vanilla Froyo running device (the G2) and injected into another, the one and only Google Phone. While this will obviously only work on N1s on the T-Mobile network, it comes as a welcome distraction to those of us waiting for the imminent Gingerbread OTA.

XDA-developers member Rsotbiemrptson has put together a short guide on installing the hack yourself, reprinted below:

Instructions:
1. Download CyanogenMod 6.1.0 rom and copy to SD-Card (Not included)
2. Download the app from the first post or this one and copy to SD-Card
3. Download WiFi_Calling.zip and extract the contents to you SD-Card
4. Flash CyanogenMod 6.1.0 (Gapps is optional, It is included in my mod)
5. Go through the initial setup and sign in, then reboot to recovery
6. Flash WiFi Calling Kernel.zip, reboot, then reboot to recovery (It's IntersectRaven's kernel but can't remember which date, fairly new though, that I have added kineto_gan.ko to)
7. Flash WiFi Calling Libs.zip, reboot (Includes libs from Evil's Mytouch4G v.0.6 Rom, and a modded build.prop from a stock 2.2.1 rom)
8. Install MS-HTCVISION-KNT20-02.apk
9. Connect to WiFi
10. Open App and enable
11. Enjoy

Rsotbiemrptson

IntersectRaven has also been kind enough to create a new kernel for this hack, which alleviates an issue whereby the WiFi connection is dropped after the screen turns off. Further information including all necessary links can be found in the thread at the source link.

Source: XDA-Developers via reddit