You can cut through a surprising amount of spam by just reading the voicemail after letting an unknown number ring. If they didn't bother to leave one it was either not at all important or the vast majority of spam calls. If they did leave a voicemail you can generally tell whether it seems even remotely legitimate and, if it does, call back at the number. E.g. if you have a kid and the school calls they will leave something generic like "It's so and so elementary calling for <your name>..." you call the school's main number back and say "so and so called looking for me" and get directed to them.

OTOH it seems if you ever answer these even once you're no longer a random number to try you're immediately marked as a real person who can be tricked into answering at least some calls and will never hear the end of the endless ringing.