Hi Braveheart53,
Welcome to the Three Community. That's a great question! I've checked with a tech expert and can confirm Three WiFi calling is only supported whilst connected to a UK based WiFi network.
You can use normal voice calls and SMS with our Go Roam whilst you have coverage on a US network. Otherwise you may wish to make use of a 3rd party app to stay in touch if you only have WiFi signal at the time.
JonathanB
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