One thing you might consider is getting a call filtering app for your phone. I use a couple of free apps, but they mostly do the trick.
I'd look for an app that:
* Uses a blacklist based on reports from other users
* Can block specific area codes and prefixes (in case a bunch of calls all come from a specific place)
* Can blocks calls from international and hidden/private numbers
* Can attempt to ID incoming calls and flag those that may be spam (one of the apps I have actually color-codes them; blue for low-risk, yellow for reported spam and telemarketing, and red for known scammers)
I'm still looking for a free app that'll block political calls and fundraising calls; those features are available with the apps I have, but they're paid functions.
Something I also recommend looking out for is what they've taken to calling "neighborhood spammers." Alfonzo mentioned spoofed numbers, and a part of that is scammers will spoof a number that appears to be from your area code and prefix (first six numbers of your telephone number), so it'll look like someone you might know and increase the odds of you picking up.
(Last note--the whole "credit card interest" robocall thing has gotten so frequent lately I've taken to just blocking all calls not from people in my contacts list until I can figure out another solution. I'm considering changing my number for the same reason.)