I get about 10+ spam calls a day, I just block all calls except for a couple of whitelisted numbers, and forward the rest to something that plays a hold song endlessly.
Sometimes I get calls from people I forget to whitelist or who might actually be important (workmen for example). So, I have my default phone ringtone set to a 0.1s, 200 byte mp3 of silence. Anyone in my address book gets my standard address book ringtone.
Then I just check the emails of the voicemails once a day.
