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=...