This requires the two phones to be on the same network. E.g. a wireless LAN or something.
Maybe you could have 1 phone act as a hotspot and the other then connects to that hospot...?
Similar. Phones have to be on a network of some kind.
Per
[URL unfurl="true"]https://www.tomsguide.com/us/zello-app-guide,news-25805.html[/URL]
"Zello is a free walkie-talkie app for smartphones, tablets, laptops and desktop computers. The app requires an Internet connection to work properly, whether it’s through Wi-Fi or a mobile data network. Unlike the standard calls you can make on the device of your choice, Zello connects you to fellow users and radio channels where large groups can talk regardless of distance.
Unfortunately, if you lose internet in an emergency, Zello won’t be of much use."
