I remember a different app thats was used on e.g. festivals where the local broadcast cells where overwhelmed when a quite rural area suddenly had to server 500

I remember a different app thats was used on e.g. festivals where the local broadcast cells where overwhelmed when a quite rural area suddenly had to server 50000 to 100000 additional people and 3g and 4G basically stopped working. I think it was called Firechat or something.