The network operator will naturally have a log of the nearest cell tower to your phone as you move around and each entry there gives an (almost, but not quite in heavilly built places because line of sight obstructions and signal bounces) circular area within which that phone was at that time (not absolutelly sure about the tower measuring and keeping logs of radiowave power levels, but if it does that circular area can be further improved to something like a torus), and the higher the density of cell towers around the phone (i.e. in cities) the smaller the areas and hence the higher location precision.

Also, at least in the US, it’s possible to get the operator to triangulate the phone’s position using multiple towers to get a much more precise location, which is how law enforcement (and who knows who else) can find people via their phones.

Even the dumbest of mobile phones can be tracked this way.