Hi llcp12,
welcome to the Apple (user-to-user) Community!
Re: Wi-Fi options for free FaceTime call from USA to Australia
I know what you mean, yet you will still use some of your wi-fi data when making FaceTime calls using wi-fi.
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Re: I am trying to make a free FaceTime call from USA to Australia.
Am no expert in this area, yet your request deserves some response.
Please see if the encouraging answer given by a "Community Specialist" in this discussion thread helps:
Can I make international calls using Face… - Apple Community
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Re: I am uncomfortable because I am afraid of a large bill.
ie: I trust you will be calling from an Apple device to an Apple device, so all have access to FaceTime using wi-fi. (Else for Apple to Android or Windows, it could be best to send an internet link for them to use for FaceTime.)
If you wish: For peace of mind:
"You can turn cellular data and roaming on or off, set which apps and services use cellular data, see cellular data usage, and set other cellular data options".
See: View or change cellular data settings on iPhone - Apple Support
NB: You can turn off (or back on later) Wi-fi Assist, which, when on, uses cellular data if / when a wi-fi signal is inadequate: See: About Wi-Fi Assist - Apple Support
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Re: "I have two options…”use Wi-Fi while roaming” or use Wi-Fi only on this phone”.
I do not know which one to turn on".
Sorry I don't know, as I haven't used those settings myself, and do not know your wi-fi set-up.
This third-party (non-Apple) Article suggests wi-fi while roaming assists when moving away from your modem / router and connecting to other nearby wi-fi access points. (ie: says: "You need to have multiple access points throughout the building" )
All the best :-) 🇦🇺