SSI is used by Survey Sampling International
They have many telephone numbers and there are many complaints on 800notes.
https://en.wikipedia.org/wiki/Survey_Sampling_International
On their website, they have an opt-out page with instructions :
https://www.surveysampling.com/contact/opt-out-do-not-call/
* Email us at
[email protected] and include your phone number in the body of the email.
* Call 1-866-360-9563 from within North America or 1-801-379-4094 outside North America to be placed on our no-contact list.
The rest of the page is worded to make it appear they are doing this out of the goodness of their corporate heart, but the real reason is that the law and regulations require them to do this.
One warning : they only have to keep your number on their internal do-not-call list for 5 years. But they don't mention that either.