The ability of calling people over Wi-Fi is truly one of the best features that T-Mobile possesses. You use this Wi-Fi calling when your mobile connection betrays you as it happens more often the server is down, then came the Wi-Fi calling into act. Most T-Mobile users have been upset about not actually having it work properly on their favourite ROM.
The users of HTC Sensation have been facing this problem for quite some time. But now, they don’t need to worry anymore as the problem has been solved for both ICS ROM’s and AOSP. The .apk file has been fixed and re-released after necessary modifications by one of the XDA Senior Member bigmoew for users to have in case their ROMs didn’t already come with it and for developers to include in their ROMs should they want to include it.
The app comes in the form of a recovery-flashable update.zip file, so there shouldn’t be any problem installing it. Though users are required to remove any previous versions of Wi-Fi calling before installing this one. You need to install it through the recovery as in includes the libraries required in order to work properly, which in case of AOSP users found to be spot on. And just because it has two versions, one for the AOSP and other one is for the stock. Though practically all sensation owners on ICS can enjoy it.
For more information check this out.
Source: xda Developers