Which is total overkill for the job. It very well might cost more to fire a Phalanx at the drone than the value of the drone. And the Phalanx has IIRC 20 seconds of fire before it needs extensive downtime for reloading.
They need something in the realm of a big automatic shotgun.
Depends on the target, some large ISR drones totally need big 20mm rounds but you scale the system down to machine guns and shotguns and build out integrated air defence systems with electronic actively scanning radar and sensors to detect and track the threats to kill
The way the US military works, throwing rolls of toilet paper at the drone from the deck of the shop is likely more expensive than the drone said toilet paper is hurling towards.
The Phalanx defense systems you see on naval vessels.