PTTR APP
Using it is extremely simple: just push the "Push to Talk" button and speak to send a message. Then listen to other person to respond.
You need to invite other persons to install the app, so you have who to communicate with.
Notes: 1. Currently the radius is about 15 km around the place where your phone is located, and this is why the GPS location needs to be activated on your phone
2. When you use PTTR for the first time Android will ask your permission to use the microphone as well as the location. The app only works if you give PTTR these permissions.