LineApp Walkie Talkie App For Your Phone
What’s different about this is you can talk to others using a local WIFI router, even without internet connection and without using cellular data. I haven’t tested this thoroughly, but based on user feedback it works. I am reviewing this app not from my own experience but giving you an idea that this technology exist and is possible. You may also look for other kind of apps that work similarly by googling it.
Check it out here on their website: https://www.lineapp.at.
