I don't think I even have the 1- for numbers I call in US, and similarly used outside of US (this was with Verizon's Wi-Fi calling, but as I recall AT&T's and basically all of them work about the same).
The "1" prefix is never needed for cell phone calls on US carriers in the US (or logically in the US). Dialing "1" before a seven or ten digit number was the old POTS Telco way of saying "this is going to be a toll call, please give me access to the toll system." (My Comcast VoIP service also does not ever require a "1" before a number.) Because so many people are used to starting phone calls with the number "1", it's never flagged as an error - it just isn't required.
What can be confusing is the US country code is also "1". But that's really just a coincidence.
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Should work as if you are calling from within US as it's basically calling from your US number.
Right - when using AT&T WiFi calling, you are logically sitting in your house making the phone call, no matter where you are physically.
Note, too, you probably want to avoid making international calls on Wi-Fi calling while abroad. Even if the call is only going 50 feet, you will pay AT&T's fairly high international rates as if you were calling from the US.
