I do this, and yeah, it tails off in a matter of weeks or days, where ignoring them just has them trying again later.

I did this on my personal phone and it seemed to work, so when my colleague and I got new work phones, I tried it again. My spam calls dropped off almost immediately, while she said she kept getting spam for months at last I heard before leaving the company.

Yes. Not answering doesn't work because it goes to voice-mail. Some will leave messages that you then need to delete which usually takes more time than just answering and hitting mute. But all will continue to flag it as an active line and will continue calling and may also sell your number to others increasing the number of calls you get.