Summary: So there is a track record on this "SSI" -- The complaints go back to 28 Jun 2014. It has identified itself as an independent research association. (See www.survey_sampling.com to opt out of getting their calls.) Then, the game switches and a robo call message is heard about real estate sales.
Most of the time, it seems this number and several others using the ID "SSI", when someone picks up the call, there is a beep, then a hang-up -- or dead air as if the line was being jammed, then a hang-up. If you wait for someone to speak, no one will speak.
What I fear is that older people, seeing "SSI", will think the call has to do with Social Security, and go into a panic. (I volunteer at a senior center, and I've heard quite a lot.)
About Area Code 504: None of these exchange numbers are for area code 504.
I think the calls are from www.surveysampling.com. I found the site when I googled "SSI + frequent calls." The last paragraph on their site gives instructions to opt out of receiving their calls. I've seen SSI's ad on web sites for looking up frequent-caller telephone numbers.