https://techcrunch.com/2017/09/10/zello-tops-us-app-store-as-the-walkie-talkie-for-hurricane-volunteers/

With Hurricane Irma hitting Florida’s southern islands as a category four storm, and more than 6.3 million being told to evacuate Florida, being able to stay in contact with the outside world is a high priority. As a result, smartphone apps which can help coordinate rescues and responses have become crucial to efforts.

The main one that’s being talked about — and downloaded at a prodigious rate — is Zello, a walkie-talkie app which was originally launched in Russia in 2007 under the name LoudTalks (but since moved to Austin, Texas in 2011) and now boasts 100 million users around the world. After being featured in a Houston Chronicle story about the “Cajun Navy” of volunteers who have been using the app to coordinate their efforts in the wake of Hurricane Harvey, the app hit the top of the US app store.