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

What about helping intercept missiles and drones? Asking for a friend.

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?