I guess emergency services must have a backup system or maybe there is a low bandwidth secondary that can't handle regular call volume and prioritizes emergency numbers.
The phone does not "dial" the emergency phone number. It tells the tower that it needs to initiate an emergency call. At that point, the infrastructure bypasses all authentication/authorization/billing checks and connects to what the infrastructure knows as the local emergency service. It's a standard voice connection, made over the same channels as a normal call, with one exception: if there is no available channel, it drops a normal call to make room.
In this case, some of the potential situations could be occurring: 1) There is still backbone (backhaul) availability, but perhaps the connection to the subscriber database has been severed. No normal calls can be authenticated, but emergency calls don't get authenticated and are thus not impacted. 2) Everything is working normally but at drastically reduced capacity. With the normal load of emergency calls, there is simply not enough capacity for normal calls. If this is the case, it is possible that not all emergency calls are being completed - the emergency operations center would not have the ability to know if this was the case.
IIRC cell towers generate temporary phone numbers and session IDs so a person in distress may use even sketchy phones without SIM or legit serial numbers to make a call for help.
I do have a recollection of watching a network outage ticket once years ago, where someone dropped their phone in a puddle which apparently caused the device to put out interference which some techs managed to track down.
There is also this case from Florida: https://www.techtimes.com/articles/161146/20160526/florida-m...
However if you have VOIP, not all VOIP providers support 911 but the ones that do want you to set a 911 address on your account itself, phone number or for a specific Analog telephone adapter - ATA Device from my understanding. I know our cable company provides phone service and the old modem had a sticker warning about VOIP and E911, not sure if the new one does or not since haven't looked at it too much.
If possible, try to use a land line, because the emergency calls are often routed to a different location when made with a cellular phone. This may not be true everywhere, but at least where I live in northern CA cellular calls are routed in such a way that response time can be 20-30 minutes slower than if made via a land line. When experiencing cardiac arrest, the difference in minutes can have a profound effect.
Apps running on the phone have access to your phone's GPS and can get more precise location data, if you have allowed it. Using an emergency app (Active911, Smart911, etc) will provide more specific location data to emergency services, but these are applications that rely on data and GPS to be working and may not work in as many situations as a simple 911 call.
As far as who receives the call, I imagine it is just based on who the tower is preconfigured to forward the call to. If the tower you are connected to is close to a highway, you'll probably get highway patrol. If you're in town, you'll probably get local police.
It's probably best to just stick with 911. That's what calltakers are used to, and trying to route around 911 means you won't get the advantages built into that system (like priority handling, mapping, etc).
The interesting implication of this is if I'm travelling in a country with another emergency number, I can "dial" the one from my home country (which is really just the sequence to initiate "emergency call") and be connected to local services.
I can see some references to 911 working on the UK phone system where the emergency number is 999 -- but this seems to be a separate mechanism (the phone company explicitly routes this number, as well as the EU emergency number 112).
It will connect an emergency call even if the quality is well below the standards for a normal voice call.
In many ways, something like an LTE tower doesn't really know what a phone call is. It just looks like packet data. So there are several fields in the signalling, that indicate that a request is related to an emergency, which should have emergency handling. This can include things like dropping other lower priority traffic, prioritizing the packets themselves on the network, etc.
But the other interesting thing is in VoLTE, you actually do a new IP assignment (to an sos PDN) specifically for the emergency call, so you have a sort of new registration to the network. This allows you to make an emergency call on any network that is available. Although scanning for an available network can take some time, so you'll likely use your own carrier if they're currently available.
Note: I've been out of the industry a few years now, and specific to emergency calls only have knowledge specific to LTE and UMTS.