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.