This app was mentioned in 3 comments, with an average of 1.33 upvotes
When I moved across the state, I used TIKL on both my wife's and my phone. It allowed us to contact each other as I was driving the moving truck, and she was driving the car behind me.
It was a simple as tapping the icon on screen, and then talking. It allowed voice activated activation as well so basically hands free. Never had a failure with it and was very easy to use.
https://play.google.com/store/apps/details?id=mobi.androidcloud.app.ptt.client
TiKL is one of the best. If you're in the US. Old school for sure, the only bad thing is that it used to work with Facebook so you could walkie talkie internationally but now that part is broken.
https://play.google.com/store/apps/details?id=mobi.androidcloud.app.ptt.client