Any of these won't generate charges from T-Mobile when directed over WiFi because T-Mobile doesn't see any of the data. When going over the local mobile network it's governed by if you get unlimited 2G web/data speeds in that country or if it's billed at a metered rate. If the connection is good enough to maintain a Skype/Facetime call, you'll be fine.

The issue comes in with WiFi calling (over T-Mobile's network) to other phone numbers. I have an international discounted calling plan, so I will check out a few corner cases next month.