Aren't you glad there is no Windows Phone anymore? Mine came with off-line maps already build in. And could use cellular masts to do a "poor man's GPS" triangulating position. Used it in this city myself (on foot), some 4 years ago, maybe 5 by now.
Same as Android's Low Accuracy Location mode - it will use mobile and WiFi networks for location, (and lower battery drain), when GPS is turned off or there's no signal.
However, it is very inaccurate if you don't have reception from at least three towers, (pretty much how GPS is useless without 3 satellite signals on cold start).
And technically it's trilateration
A bit more trivia:
Triangulation would require two receivers at two different locations, (or one receiver moved between two locations), because it uses angles to get the distance from the towers.
Trilateration only requires one receiver at one location but uses three transmitters. It uses the broadcast transmitter position along with signal strength, where the transmitter radio spheres intersect is your location.
If you've ever logged your GPS track and lost GPS coverage but still have mobile coverage from one tower, you'll see that the track will sometimes jump to the location of the tower.