Zello was downloaded one million times in
The demand is so great it has added 18 new servers, it confirmed.
However the firm has had to warn users that it will not work without either a mobile data signal or wi-fi and that it will drain smartphone batteries.
Zello is currently the most downloaded free app on Apple's US App Store, and claims to have had more than 110 million downloads across Android, iPhone, Windows, Blackberry and PC platforms since its relaunch in 2012.
