The current Airwave TETRA based system is capable of point to point radio comms should the main network become unavailable.
The replacement 4G based system does not have that facility - no network, no comms.
Go out of coverage, you're on your own.
Emergency Services is, by its very nature, not always going to be in a controlled "everything works" environment.
Just looking at the fires in Los Angeles where Musk has put StarLink wagons in there to provide a mobile telephony uplink illustrates how susceptible to disruption civilian comms systems can be.
The whole concept of ESN was flawed from day one and illustrates the inability of those in government IT procurement to be trusted with anything more sophisticated than a torch.
This stupidity has been going on for a decade now and we'll be having the same discussion in 2033.