ArjanBThank you everyone, I think I have what I need. What I figured out was by entering that code above (I'll enter it correctly here: * # * # 3642623344 # * # * [no spaces]), it makes the option available, and it will make it searchable in settings. I guess Mint (or T-Mobile, not sure which one) hasn't approved the ZF9 for Wi-Fi calling features yet (after speaking with Mint mobile tech support). According to ASUS, Wi-fi calling is "not available for unlocked devices," which is surprising, because I though the ZenFone 9 was basically an unlocked device.
T-Mobile "calling over wifi" works in my country, so it should only be a setting change for Asus.
The other provider it does not in my country is KPN.
Entering the code #*#3642623344#*# does bring up "calling over wifi" for the KPN, but it doesn't work