Dont answer calls I dont know number, if collection agency they should leave message, if no messages are left I do not feel they are important and always reverse lookup numbers.
I have been getting this phone call a bunch of times from different numbers for weeks now and it's driving me crazy. It's a total scam and they make it automated so you can't scream at them to stop calling. I have been on the Do Not Call List since 2004 and they obviously aren't complying with that.