Zello, a walkie talkie app, was downloaded more than 1 million times in a single day, mostly by people in Puerto Rico and Florida preparing for Hurricane Irma, after success stories of the app being used in Hurricane Harvey relief efforts.

The app’s user numbers now total 100 million and, according to CEO Bill Moore, it is number one app in the US App Store and number 30 on Google Play as a result of “press stories covering Zello’s role organising citizen rescue efforts in Houston.”

However, founder Alexey Gavrilov pointed out in a blog post: “While Zello has been helpful in Harvey relief efforts, it is not a hurricane rescue tool and is only as useful as the people who use it, and as reliable as the data network available”, adding Zello will consume a lot of battery power when used.

On the plus side, he said: “Text messaging apps and Zello use a fraction of the bandwidth of phone calls and will often work when phone calls won’t get through.”

Zello describes itself as a free push-to-talk (PTT) application for smartphones, tablets, and PCs, which is lightweight, easy to use and extremely fast because users don’t need to dial a phone number or send a text. Instead they hold down a button to speak to each other live, much like a walkie-talkie.

“We don’t like bloated software. Zello setup is lightweight and the program uses very little system resources and memory,” the company said on its website, adding: “Zello supports channels where you can talk to one person or up to 1,000 people from all over the world at the same time.”

The app began in Russia in 2007 as Loudtalks for Windows PC, and then relaunched as Zello five years ago in Texas.

Althouth the app racked up 400,000 active users in Russia, it is now blocked after the company said it is unwilling and unable to provide the government with user data, CNBC reported.

Zello also offers a version for businesses called ZelloWork, which offers management tools, privacy, and security options.