Assuming you have an operator that supports wifi calling and a phone that both supports it and is "whitelisted" (basically USA, Europe does not do such silly things)
woosh - read the comment again, it clearly implied the US has a prominent idea of whitelisting "compatible" handsets (ie; those bought from the network because unlocked is a hilariously quaint idea) whereas in Europe, unlocked handsets are generally the default, since people don't like it and regulations prevent it for the most part anyway, in those cases it's just incompatible/old/awful implementations, rather than operators denying said features.
But just in case: the quotes emphasised the act, the parenthesis referenced said nonsense with the target being the first mentioned (it could not possibly be interpreted any other way) as a result of the parent comment likely being from the US as T-Mobile outside of the US does not refer to itself as such, it's either what it became (ie; EE in the UK) or a regional variant.
You really gotta dial down the smugness. It made your comment difficult to actually understand and so annoying that I didn't want to finish reading it. Take whoosh back to reddit, please.
Yes they can? I have no cellular service at home but I have wifi, and my iPhone connects to "T-Mobile Wi-Fi" via my home internet.
SMS messages are sent and received just fine.