The T-Mobile myTouch 4G comes with it, and the T-Mobile G2 just got an update to enable it… what is it? It’s the ability to make and receive phone calls over a connected WiFi network rather than over T-Mobile’s cellular network.

First off, why would you want to do this? It’s only good for T-Mobile users, and if you’re not on an unlimited minutes plan, WiFi minutes still count against your plan’s minutes. You don’t get a discount on your bill. You have to provide your own WiFi. So why would you want to do it?

Simply put, sometimes your cellular signal isn’t that great, or you might be roaming (at a friend’s or relative’s house). In these cases, if you’ve got a strong WiFi connection, you can still make and receive phone calls to and from your T-Mobile (or Google Voice) number without changing the way you dial, or having to launch another app — just as long as you’ve set up your WiFi calling. Unfortunately, it’s not available on every Android phone, and if you’re not on T-Mobile you won’t be able to use it.

The “you have to be on T-Mobile” part we can’t help you with, but if you’ve got a Nexus One and are willing to do some hackery, you’re in luck!

The standard disclaimer applies: we’re not responsible for any damage that’s done from following or attempting to do this… you’ve been warned.

Next, you’ll need to unlock your Nexus One, throw on a good recovery image (like RA’s Recovery), which we’ve told you how to do in the past (if you need help with this, use our Site Search to find out how).

1. Download CyanogenMod 6.1.0RC1 and copy it to the root of yoursdcard (you can grab a copy from forum.cyanogenmod.com)

2. Download the WiFi calling .apk file from the post at XDA-Developers and copy it to the root of your sdcard

3. Download WiFi_Calling.zip file and extract the contents to your sdcard

4. Flash CyanogenMod 6.1.0RC1 (and optinally the GAPPS package, also available from forum.cyanogenmod.com)

5. Go through the initial Android setup and sign in

6. Reboot into recovery

7. Flash WiFi Calling Kernel.zip and reboot

8. Reboot into recovery again

9. Flash WiFi Calling Libs.zip and reboot

10. Install MS-HTCVISION-KNT20-02.apk

11. Connect to your WiFi

12. Open the WiFi calling app from your app drawer and enable WiFi calling

You may want to change your WiFi advanced settings to keep WiFi on even after you’ve turned off your screen, otherwise your WiFi will turn off and you won’t be able to receive incoming calls via WiFi.

Source: XDA-Developers