T-Mobile now supports Wi-Fi calling on iPhones with iOS 8 and higher. This is a big deal to:

1. People who live in the U.S. but travel abroad
2. People who live outside the U.S. but have a VoIP (like Vonage) line back in their home country, and/or have U.S. SIM cards for their travels to the U.S.

Since I know a lot of people in the first group (including myself), and a lot of people in the second group, I feel like explaining:

If you have a T-Mobile line, you can use it from anywhere outside the U.S. for voice calls to U.S. numbers as long as you are connected to Wi-Fi. No roaming charges apply. You can receive calls too. See this FAQ.  And as before, you get cell-phone data and SMS messages.

And when you are in the U.S., you can of course use the cellphone normally. Including using data and Personal Hotspot. Or texting from airplanes at no additional charge. This will work on iPhone 5c, 5s, 6, 6 Plus (this has existed in other phones since before iOS supported it).

@marcoskirsch The iPhone 5s and 5c will also support Wi-Fi Calling with the launch of iOS 8. 🙂 *MG

— T-Mobile Help (@TMobileHelp) September 17, 2014

See the following screenshot: phone is in Airplane Mode but with Wi-Fi, and I’m making a regular phone call to a regular phone line. To use this, upgrade to iOS 8 and enable in Settings -> Phone.

This is really outstanding.