To use Wi-Fi calling with T-Mobile, customers must have a supported T-Mobile Wi-Fi Calling-capable phone and Wi-Fi Internet access. As of March 2015, this mobile phone carrier doesn't work with external apps designed for Wi-Fi calling.
When using T-Mobile Wi-Fi calling, no charges apply for either outgoing or incoming calls and texts while within the United States. No charges apply for texts apply when roaming in a Simple Global country, but texts sent from a non-Simple Global country carry a 50-cent charge. If roaming in a Simple Global country, Simple Global rates apply for Wi-Fi calls, and World Class call rates apply on Wi-Fi calls from non-Simple Global countries.