EDIT: Sorry, I am idiot. What I described IS triangulation.
Reeeeeee! Phones. Are. Not. Triangulated.
Most cell towers use phased antenna array, so they know relative direction all the time. And distance can be estimated from latency and signal strength.
Two cell towers allow to get precise location from angles. Angles are derived from phase differences on elements of array and can't be manipulated like latency or signal strength.
What do you think the cell towers are triangulating and forming angles with? The person's phone is the third point in the triangle. You only need two other points to triangulate something's position.
I checked it. Damn. You are right. Point with unknown coordinates counts. Now I have to edit my comment and answer other subcomments of people I confused.