Ok for the record, I'm an iOS user (yeah, boo for many of you) and found this question because I wanted to know if I use WiFi Calling on iOS8, if it uses minutes in my pre-paid account.
After reading through this thread and seeing ppl comment on all kinds of apps like GV and Skype, I also want to emphasize i believe the original poster is referring to the T-Mobile feature called "WiFi Calling" so this is what I'm going to speak to & feel answering the question is independent of hardware (ie. whether you are Android or iOS) - I just couldn't bear reading another GV-related comment here because as much as I love GV, i feel it's *off* topic.
So I just enabled WiFi Calling yesterday - while I havent engaged in an actual call, this is what I can add:
1. My phone says "T-Mobile WiFi" at the top confirming its enabled
2. I can put my phone in Airplane Mode and incoming calls ring the phone & provided I don't answer it, it doesn't not use my minutes (this is awesome for screening calls & avoiding the daily usage charge in many circumstances btw)
3. I received a survey from T-Mobile by text (not iMessage) and was able to complete the survey (interactive chat) on "T-Mobile WiFi" - just checked by balance and it did not charge me for the day (nor did I expect it to for T-Mobile business matters!)
So the next step I need to take is to actually make and receive a call and exchange a few non-iMessage texts with a friend while in Airplane mode.
My gut says it will work just fine & I'm one happy camper!
Btw - for you google voice users masking your SIM #, you need to turn off "Show my Caller ID" on iOS because it does not automatically spoof your caller ID to your GV # (like i'm aware Android can)
Again, I'm trying not to muddle up the waters with iOS talk - I'm just wanting to contribute to the poster's original question in regards to using "WiFi Calling" over T-Mobile prepaid.
It sounds like from a few of you, the general concensus is "if you are trying to make a call through T-Mobile (not GV, not Skype) whether you are funneling through cellular towers or through 'T-Mobile WiFi Calling,' it uses minutes in your plan. End of story."