Zello was quite useful to many people before, during and after Harvey and Irma. Is it going to work in all situations, if there is zero cell signal? Nope.

Fortunately, the cellular network can be surprisingly resilient, as my brother found out as his cell phones kept working throughout Irma's assault on Naples.

https://www.newyorker.com/news/news-desk/listening-to-houstons-desperate-cries-for-help-after-harvey

By midday today, some of the volunteer boat rescues in Texas were being coördinated on a walkie-talkie app called Zello. Trapped people unable to get through to a 911 operator were trying Zello’s “Cajun Navy” or “Texas Navy” channels, and asking for help from citizen dispatchers and boaters. Early this afternoon, the requests for help—most of them less than thirty seconds—were relentless; there was rarely any dead air.


http://money.cnn.com/2017/08/28/technology/volunteers-harvey-rescue-apps/index.html