Radio 26.13.04.19 required for this to work properly.
IMPORTANT UPDATE: Update to Version 2.1 Why? I have done some research on flashing radio images and found some interesting information. Apparently there are 2 files that pertain to radios in the RUU releases, a radio.img and a rcdata.img. We all have been flashing only the radio.img file but the rcdata.img contains further carrier information that directly relates to information in the build.prop file. I have put together a new 26.13.04.19 radio zip file that contains both the radio.img and the rcdata.img. The .img files were taken directly from the latest T-Mobile RUU release. This new radio/rcdata zip needs to be flashed through bootloader just like you flashed a radio in the past. As usual, you''ll need to rename the zip file to PC10IMG.zip before you reboot into bootloader. Check the MD5 Sum after you download the file to your sdcard to make sure it matches the following:
MD5 Sum: 360EA26286DEC39FC8008B266F470839
I have also updated information in the build.prop patcher with the proper information to match the information in the rcdata.img. This is now Version 2.1. After you flash the radio/rcdata img from the above link through bootloader, download the latest version 2.1 and flash through recovery along with the EFS wipe.
Update: Some users have reported that they have received SIM card related error messages. I highly recommend upgrading your SIM card to the latest TM9177 version if you are running any other version SIM. The 9177 version is what is currently shipping with the newer T-Mobile devices pre-loaded with ICS operating systems. I have this SIM card installed in my G2 running the latest Andromadus test builds by Flinny and it's working perfectly with no errors.
I have put together a working T-Mobile Wifi Calling Add-On for Vision devices running the latest CM9/Andromadus/AOKP ROMS and the latest T-Mobile radio firmware 26.13.04.19. I confirmed this working with the latest T-Mobile .19 radio. This add-on will also update your radio lib-ril and gps .so files for better performance and faster gps locks without experiencing the "gps signal lost" error message.
If you are not running the .19 radio along with the latest rcdata.img, download from the above link. If you need further flashing instructions get them here:
This version includes the patch to the build.prop and all files included in Version 3.1 of the lib-ril and gps .so updater for the .19 radio. Once you have updated your radio/rcdata then flash the Wifi Calling Add-On. It is also very important that you flash the Vision EFS wipe after you flash this add-on since you are updating your radio drivers. You should always flash the EFS wipe if you have booted your system with different radio drivers.
Download Version 2.1 Wifi Calling Add-On for Vision.
IMPORTANT: While you are in recovery, wipe your cache and dalvick cache and wipe the efs partitions by flashing the Vision EFS partition wipe tool.
Download EFS Partition Wipe Tool
Feedback and Thanks are always welcome!