Everyone's been asking about it. Everyone wants it. Here it is, the wifi calling app from the G2! I've installed and runs perfectly fine on my vibrant running JP3 froyo 2.2. My home network is weak so i can't really test yet. Kudos to @Official_Drizzy for the .apk
Install with your preferred method of install, I used astro just fine, or push it to /system/app via adb. Whatever floats your boat.
If you have a non-T-Mobile phone and are a T-Mobile customer, you simply dial 611 and ask them to activate the service on your account. It's not necessary on phones issued by T-Mobile because Wi-Fi calling activation happens automatically. If the tech knew what he was talking about, there is no software to add now, as it's a provisioning issue. The tech said their provisioning window is about two hours, and if the setting for enabling Wi-Fi calling did not appear in Settings>Connections>More Networks (my phone runs Android 4.4.2, and the path to the settings is a little different on earlier versions, but easily found) within 2 hours to reboot the phone and it should appear. Time will tell.
UPDATE: Yeah, time did tell. The setting didn't magically appear, and when I called back after the provisioning window, got escalated to a second-tier tech who said that it's still baked into the firmware of their branded phones and can't be added to an unlocked phone. So I read them the riot act and told them how stupid it was for them to base their entire advertising strategy on courting customers away from other companies and then not making features like their wi-fi calling available to them. Oh well.