Cell Towers Can Double as Cheap Radar Systems for Ports and Harbors (2014)

Sure, but not as well as a dedicated radar system and at much higher cost. In TFA they spotted small speed boats at 4 km, and needed a trailer full of RF equipment. Drones and missiles would be detected even later, since the antennas of cell towers are designed not to radiate any energy upwards (there's usually no cell phones high in the air, so that energy would just be pure waste).

You might be interested in a similar system in Ukraine that uses a huge amount of acoustic sensors (basically just weatherproofed microphones) to detect the very loud engines of Shahed drones as they fly by, and then directs air defense crews based on that approximate location data.

how dos this acoustic system work, and how does it distinguish drones from other noise? Does it use any form of AI?

Drone propellers make a distinctive noise, which can be isolated quite well from background noise with FFT analysis. I doubt they'd choose to use AI for that, as classical methods work perfectly fine and need much less processing power.