I don't know how it is with VoLTE and other recent things (it may be the same; it may be different), but at one time in cellular world: Emergency calls differed from other calls in that they Must Always Work.

An emergency call can connect using any tower that is compatible with the caller's hardware -- with or without service provisioned, and with or without any sort of SIM.

Need help, and find a dusty phone somewhere? Turn it on, call emergency services using 0118 999 881 999 119 725 3 [*], and if there's any cell service within range that it is physically capable of chatting with then the call will go through.

It will kick other users off if that's necessary in order to allow the emergency call to happen.

There's nothing to bill, so there's no billing systems (or even billing logic) to get in the way either.

It's intended to always work.

*: https://www.youtube.com/watch?v=HWc3WY3fuZU

Namely the fact that emergency calls can be routed through other networks that aren’t your own (in fact, you can place an emergency call without a SIM).

You can also make emergency calls when you don’t have enough signal for regular calls.

It forces it through