[Galaxy S 4 Developer Discussion [Developers-Only]] Fenny:
This thread is as much a dumping ground for what I figure out, as it is an information sharing endevor. Anyone who feels that they can contribute, feel free.

My goal is to take the tmobile wifi calling code, and make it portable to as many devices/roms as I can. Ideally I would like to take whatever I need from the various frameworks, and make it portable enough that we could inject it into even cyanogenmod with little to no hassle.

All of this is based on the m919 t-mobile version of the SGS4.

This is what I have figured out about wifi calling so far:

Files Containing Wifi Calling stuff

./app/ims-service.apk -- whole file? possible merge required.
./app/ip-provider.apk -- whole file
./app/IPService.apk -- whole file
./app/SecContacts.apk -- We REALLY should merge. This may be easier to just copy to other TW Roms.
./app/SecMms.apk -- We REALLY should merge. This may be easier to just copy to other TW Roms.
./app/SecPhone.apk -- We REALLY should merge. This may be easier to just copy to other TW Roms.
./app/SecSettings.apk -- We REALLY should merge. This may be easier to just copy to other TW Roms.
./app/WifiCall.apk -- whole file
./framework/jsr-api.jar -- partial methods + merge -- This contains a class that we can simply copy into our target framework, but there are still files that must be manually merged.
./framework/services.jar -- merge
./framework/telephony-common.jar -- partial methods + merge -- This contains a class that we can simply copy into our target framework, but there are still files that must be manually merged.

Files needing to be merged.

./telephony-common.jar/com/android/internal/telephony/gsm/GsmServiceStateTracker.smali
./telephony-common.jar/com/android/internal/telephony/PhoneFactory.smali
./telephony-common.jar/com/android/internal/telephony/CallManager.smali
./telephony-common.jar/com/android/internal/telephony/uicc/SIMRecords.smali
./telephony-common.jar/com/android/internal/telephony/uicc/IsimUiccRecords.smali

./jsr-api.jar/com/android/ims/core/media/player/RecvPlayerImpl.smali
./jsr-api.jar/com/android/ims/core/media/player/RecvPlayerImpl$1.smali
./jsr-api.jar/com/android/ims/core/media/player/SendPlayerImpl.smali
./jsr-api.jar/com/android/ims/core/media/player/SendPlayerImpl$1.smali

./services.jar/com/android/server/WifiService.smali

./app/SecSettings/apk/res/values/strings.xml
./app/SecSettings/res/xml/wifi_calling_settings.xml
./app/SecSettings/smali/com/android/settings/deviceinfo/Status$6.smali
./app/SecSettings/smali/com/android/settings/deviceinfo/Status.smali
./app/SecSettings/smali/com/android/settings/wifi/WifiSettings.smali
./app/SecSettings/smali/com/android/settings/SecuritySettings.smali

./app/SecContacts/smali/com/sec/android/app/contacts/util/ImsLowSignalHelper.smali
./app/SecContacts/smali/com/sec/android/app/contacts/util/ImsLowSignalHelper$1.smali

./app/SecPhone/res/xml/wifi_calling_settings.xml
./app/SecPhone/smali/com/android/phone/GsmUmtsAdditionalCallOptions.smali
./app/SecPhone/smali/com/android/phone/CallController.smali
./app/SecPhone/smali/com/android/phone/GsmUmtsAdditionalCallOptions$3.smali
./app/SecPhone/smali/com/android/phone/InCallControlState.smali
./app/SecPhone/smali/com/android/phone/InCallTouchUi.smali
./app/SecPhone/smali/com/android/phone/NotificationMgr.smali
./app/SecPhone/smali/com/android/phone/InCallScreen.smali
./app/SecPhone/smali/com/android/phone/CallFeaturesSetting$16.smali
./app/SecPhone/smali/com/android/phone/CallCard.smali:
./app/SecPhone/smali/com/android/phone/PhoneUtils.smali
./app/SecPhone/smali/com/android/phone/InCallMenu.smali
./app/SecPhone/smali/com/android/phone/InCallScreen$38.smali

./app/SecMms/smali/com/android/mms/util/IMSUtils.smali
./app/SecMms/smali/com/android/mms/ui/ComposeMessageFragment$1.smali
./app/SecMms/smali/com/android/mms/ui/ComposeMessageFragment.smali

More to come as I figure out more.

*EDIT*

I am working from the open source project here:

https://github.com/FennyFatal/the-ims-open-source-project-for-android