They know its a working number if you pick up the phone so asking to be placed on the do not call list won't matter but I would not give them any other personal info if I did.  

As far as some people saying that they answered but the other end hung up:  Absent the very occasional incident where a real person happens to give up and hang up just as you pick up the phone, call centers do this to build a data base of real phone numbers that they sell to other "telemarketers."  Sometimes this is legit, other times its pretty slimy.  The autodialer is just looking for real phone numbers.  If the phone is answered and not a fax--number goes in data base.  It might anyway if they can find it cross-listed so not answering is not likely going to keep you off the list. They hang up once you pick up because the job is done...for the moment.

I have a friend who works on a college poll.  She says that she buys lists of 15,000 random numbers to make a public opinion survey.  This happens about twice a year.  There's her school and one other that polls in Maryland as far as I know.  The Washington Post and Balitmore Sun might do this a few times an election year.  There are two (real) private firms she knows of that do this very occasionally--one does the work for the Baltimore Sun the other does about one a year pro-bono and occasionally is hired to do one by a political campaign.  15,000 sounds like a lot of numbers but it is 15,000 out of more than 4 million registered voters.  She looks to connect with 1,000 out of this.  The odds you will ever get a call from her is about zero and if you do make the cut, you might get six to seven calls to try to find you at home.  After that , the survey is done (in about 3 days) and the list is trashed.  This is typical of all real opinion research organizations.  Next survey, new sample.  So, I am explaining that legit "research" firms aren't likely to be known for making a lot of calls and you might get a call (6-7 if you don't answer) from one in your entire lifetime.  SSI is really a telemarketer I suspect running a call center.  They may be one of the ones that registers in name as an opinion research firm in order to skirt the laws or operate in a grey area under the guise of a research firm--but I would guess most of their work is solicitation of funds. "Real" researchers call this push-polling and it makes everyone's life more difficult as they pester people such that they refuse to talk with real researchers (when and if they do call).