BLUE: This is an iMessage. iMessages use data because they route not through to a phone number, but to an Apple ID that is associated with that phone number. This is why you can message on WiFi but not data. GREEN: This is a Text Message. You can only send these through the cellular network and not through WiFi.

Jan 16, 2014 Did T-Mobile stop charging voice minutes for calls over wifi? Nov 22, 2013 WiFi Assist breaks WiFi Calling? - Apple Community

Use your phone however you want, whenever you want with unlimited incoming and outgoing wifi calling, voicemail, and text messaging. View Pricing Relocating, moving for work, or studying overseas does not mean that you should lose touch with family and friends.

Because he said in the presentation that ALL Tmobile customers would have access to free WiFi calling with a supported phone. Unforutunately, there was no * to denote while there is no monthly fee to be able to use it, it does use your minute allotments. No it's always been this way. Idk why that was even a question. - /u/mellybiird Make calls over Wi-Fi - Google Fi Help

Simply Prepaid plans | T-Mobile Support

Does T-Mobile charge for WiFi usage on your phone? | Yahoo