One use case is the GSM network ran on Chaos Communication Congress hacker conference in Hamburg. They created an internal network for the attendees to - among

One use case is the GSM network ran on Chaos Communication Congress hacker conference in Hamburg. They created an internal network for the attendees to - among other reasons - communicate for free: https://events.ccc.de/congress/2015/wiki/Static%3AGSM https://events.ccc.de/congress/2015/wiki/Static%3AGSM