At least one carrier needs to have coverage in the location you are trying to make an emergency call from. If none of them do, your cellphone won’t be able to connect an emergency call. It’s not magic, just engineering.
The physical components for connecting to cell phone towers of various carriers and various tech are mostly the same. The difference comes in which cell towers you tune to. A cell phone connects to a tower by “tuning” into what are called bands or channels. For something simple as GSM, it is literally frequency bands. For newer tech like CDMA and LTE, it’s a bit more complicated because the tower signal jumps frequency bands and is mathematically encoded.
When you buy phones from different carriers, they are set to ignore bands or channels that the carrier doesn’t use. It serves several purposes
- to make sure you can’t take to phone to another carrier
- to make sure your phone doesn’t waste time and battery tuning to cell towers that won’t accept them (if you have an AT&T service, the T-Mobile tower will reject you even if you can talk to it)
- Less testing of the phone necessary, etc. There’s software that picks the best tower out of all the towers your phone can hear/talk to (you might be able hear/talk to multiple towers from your carrier, you want to pick the closest one or the one that supports the fastest data, etc.).
When you make an emergency call, the software that’s responsible for picking to tower tells the tuning software that you are making an emergency call. The tuning software also keeps track of the last few bands/channels in which it saw any signal. It’ll try them first so that you can connect the call quickly. If it can’t find any, it’ll do a full sweep of all the bands the physical hardware can possibly tune to.
Once it finds a tower, the software for making calls asks to connect as an emergency call.
The tower will then connect you. If the tower is at full capacity, it’ll kick out someone who’s not in an emergency call and then connect the incoming emergency call. This is very important when used in large crowds.
In case your call disconnects, you phone will connect back to the same tower again. This is so that the carrier can try to locate you using the tower signals. If you had connected to another tower, the emergency people might have to connect to the different carrier and start locating you all over again.
Each country has a different emergency call number. It’s 911 in the US, it’s 112 in Europe, and 999 in the UK. What if someone goes to a different country and get into an emergency? They won’t remember what that country’s emergency number is — so the software in your phone will see you are dialing 911 in Europe and go “this person just wants to make an emergency call. I’ll just connect to 112 instead”.
Credit to Reddit user not_anonymouse