When they called last month, I added them to my phone book as "Survey" and so did not bother to answer when they called back yesterday.  

If you call them back, you won't be able to speak to a live person.  I guess the only way to get on their do not call list is to go to their website and send them an email.  

Sadly, these "survey" calls are legal.  However, once you tell them to stop calling, they cannot call you again.  If they do, they can be reported to the donotcall.gov website.