An app that enables people to have live conversations in large groups is being used as an unofficial rescue co-ordination tool during the US hurricanes.
One woman shared a harrowing account of her experience collating people's SOS messages received via the app and dispatching volunteer rescue boats.
Zello was downloaded one million times in one day, the firm said on Facebook.
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.
"It is not a hurricane rescue tool and is only as useful as the people who use it," the firm wrote in a Facebook post listing tips for using the app during a disaster.
"If your phone battery is under 30%, turn off Zello. That way, it won't use any power, and if someone sends you a message, you will still receive a push notification," it added.
Journalism teacher Holly Hartman wrote a Facebook post detailing her time co-ordinating rescue efforts by volunteers with boats, known as the "Cajun Navy", in response to distress calls put out on Zello during Hurricane Harvey in Texas because the emergency services were swamped.
Ms Hartman described hearing from families with young children who were trapped in their attics with rising water, a young woman concerned about her 87-year-old grandfather, and a teenager whose brother and cousin had died.
"Voice after voice after voice coming though my phone in the dark, some asking for help, some saying they were on their way," she recalled.
"Call after call from citizens saying they were trapped in their houses and needed boat rescue."
Analyst Jack Kent from IHS Markit said that voice apps have clear advantages over text-based communications platforms during emergencies.
"They can allow hands-free communication and the ability to broadcast information to large groups and public channels," he said.
"This differs from the text-based more limited, direct and smaller group communications of other messaging apps."
The Japanese messaging app Line, which has around 169 million monthly active users, was developed following the Tohoku earthquake in 2011, he added.
Tesla increases battery power in Florida.
Car giant Tesla remotely unlocked the battery capacity on its Model S and X 60/60D vehicles in Florida to enable drivers fleeing Hurricane Irma to go further between charges, the firm confirmed.
The battery capacity on these models is usually locked down unless the owner chooses to pay for extra power.
One Model 60 owner in Florida told the website Electrek that he had noticed 40 extra miles in his full charge.
The firm said it had temporarily put the measure in place using its wireless software update system.