You have to look at the issue from the organizer PoV and not as the event goer. You need to have some kind of reliable communication channels for the staff. The result of 10 years of experience of trying to cause that is for me that analog UHF radios work, wired VoIP phones work and just using whatever smartphones the staff works and combination of these three technologies gives you the SLA level you need. DECT is meaningful solution of the same problem, but the handsets are expensive and even more so if you want some broadcast/PTT capability and ideas like “well, we have bunch of Nokia S60 handsets that should in theory support wifi as LTE/CAE radio layer” is pure nerding-out that does not solve any kind of real issue and instead creates new ones