SSI is Survey Sampling International. They conduct opinion polls and market surveys for a variety of clients including political organizations. They have numerous call centers in several states and foreign countries. Because their calls do not lead to a sales attempt nor do they provide your number to anyone as a sales prospect, they are not required to screen against the DNC list before calling. They are required to maintain an internal Do Not Call list but apparently the only way to get placed on it is to follow the opt-out instructions on their website:
This has been used to call us more than a few times. I have it logged many times... A message is never left as we don't answer calls we don't know or recognize !!