They operate several different networks for voice communications during the events. From what I found usually DECT, SIP, GSM. [1] They have a status dashboard with metrics during the events. [2]

[1] https://events.ccc.de/2024/12/22/38c3-poc-isdn-version/

[2] https://dashboard.eventphone.de/d/de7sgxz63vzeoe/38c3?orgId=...

Yes it's pretty handy for hacker camps. I still have one too. Especially because out on the camping fields the WiFi coverage can be hit and miss. This way you can even get calls when you walked to the toilet building or the car park.

Also it's got its own frequency so it's not cluttering the ones used for WiFi, Bluetooth, Zigbee etc.

I guess they could use an app or something but dect is rock stable and has much better range than WiFi.

I got a nice Siemens one that is about the size of a nokia 8210 so it's not like you have to carry a huge brick either. I guess the battery is pretty dead now though. But it is replaceable like all batteries of that time.

DECT is still extremely popular within (private) houses in Germany.

Way more robust than VoIP over Wifi.

I think it's a CCC tradition.

Yes, although you can also use GSM or SIP and it's often called a DECT number even if you're actually on one of the newer networks, since the number space is the same. DECT simply was there first and thus became the generic term for the internal phone network.

There is a conference phone network hosted by eventphone. The most popular way to access it is indeed using DECT handsets, but you can also access it via SIP (probably the second most popular method), GSM (yes, really, they used to hand out SIM cards), landline (supports rotary phones!) and probably even more esoteric ways. There's also a post office delivering mail both internally and externally.

Don‘t they even have an ISDN network there this year?

Yes, I was looking at the conference website before it started and they still operate DECT phones.