I believe the use of the data is not charged by either the parent carrier (T-Mobile For Me) or roaming carrier (Say, Vodaphone In Europe) because the data required is so low.. The amount of bandwidth required is minimal. I have tested it and it seems to be around 16kbps, or 1Mb per minute on a voice call. The free 128kbps or 256kbps can easily handle these type data requirements. You can even do low resolution FaceTime with that. Also, it apparently uses similar technology built into iPhones at least that is employed for Facetime Voice calling.

I'm guessing that it does not require a voice channel from either carrier, so it cannot contribute to tower voice congestion.

Edited to add: I see I really didn't answer your final question about the possibility of a WiFi charge for calls. WiFi calling will incur a charge if you are connecting with a non US number. Or, if you get a call from the US and have WiFi on, it might connect via WiFi calling @ $.25 a minute.