There are tons of call center farms with people packed very closesly using Bluetooth headsets (and have been for a decade) and there doesn't seem to be an issue
Yes, they are likely DECT, my last job we used Plantronics CS500 headsets which are in fact DECT. Though, regardless of the protocol used (DECT 6.0 uses 1.9GHz) congestion is always an issue with that many devices in close proximity, especially at the range I can take DECT headsets - in fact, this is why I bought a CS500 for home use along with the EHS adapter for my Polycom Soundpoint IP 335, it's nice that I can be on a conference call for work and go out to the garage to grab something quick without having to carry my phone in my pocket.
If you meant something like this [0], DECT [1] works on a different frequence band.
[0] https://en-us.sennheiser.com/phone-headset-wireless-call-cen...
[1] https://en.wikipedia.org/wiki/Digital_Enhanced_Cordless_Tele...