The search space of valid numbers is very small compared to email. A throw away number is surely a number that has already been trashed by dozens of users before you. It’d be useless unless you can make everyone forget it between users.

My personal cell number is 20 years old and gets almost no spam besides a rare robo call. Any throw away number I’ve acquire has a sordid history of being used as a honeypot it seems because it’s a cesspool.