how do i call to sydney australia from bozeman montana on an iphone12

She would have had to properly set her international calling up with her cellular carrier at home before leaving.

About cellular data roaming options for your iPhone and iPad - Apple Support

Or perhaps she's in an area where the cellular carriers (Australian and American) don't cooperate, but that's unlikely since she's in Sydney (unless she uses a more obscure American carrier).

If your friend has a cellular telephone, call your friend just as if (s)he were still in the U.S. Your friend will pay extra, of course, to have that call communicated down to Sydney.

Sydney's 1-digit code is "2" for a landline, "4" for a mobile line.

What happens when you dial the number?

You're welcome. Enjoy your Apple gear.