It depends on the area. In very dense areas, it can be more accurate than you expect. It's also possible that they triggered a GPS response that told them rather precisely where the phone is.

I'm only speculating, of course.

A few years ago, when I was studying, my friend and I did a project on that in Xamarin. It took us two evenings, and the accuracy was 300 meters at worst, and 30 meters at best. It's worth noting that this was in a larger city.

So, if two students can achieve even 30 meters accuracy in two days, big telecom corporations will certainly do better in a few decades.

EDIT: By the way, it wasn't only triangulation but also signal strength change analysis. It wasn't as serious as it sounds, there are formulas already made for calculating that.

In the past, ~1 mile. One of the complaints people had about 5G was that it could make it much more accurate. Accurate enough to find a house or someone's location with it. It works because 5G needs towers all over the place to function.

Did the friend park the car out the front of his house, with a license plate that would have been shared by the sister? In the suburbs you don’t need to be overly precise, do a drive by and stop at the house with the car?

E911 says the PSAP has to be able to get within 100m iirc

This is about asking the cell network where the phone is. This is not about asking the phone where the phone is.

I think the question was "how did the phone company know the location", and GPS may be part of the answer.

In the US, E911 requires all phones to be able to report their physical location. Phone companies may use this ability to respond to police location requests. I don't know one way or another, but it seems likely.

Do phone companies have access to that when the phone isn't actively placing a 911 call?

Even if they don’t, they can use trilateration/triangulation (for which I assume they always have the necessary information: signal strength and approximate distance) to pinpoint the exact location.

Good question. Big picture, the phone companies have access to that information any time they want.

The E911 laws, though, only require that the location information be obtained and forwarded when the call is placed.

So, I don't know. I don't know anything beyond that.

They won't use that if the GPS / "Location" service is turned off. But the cell network still knows where you are anyway, within ten meters or better. The FCC asks for z-axis accuracy within 3 meters; good enough to determine what floor of a building you're on (for E911 purposes.) 5G Rel 17 can supposedly locate people within a meter.