The caller ID says "SSI"  but I don't recognize the company name or number so I don't answer. Whoever this caller is never leaves a message and they call frequently.  The number is now blocked.

SSI = Survey Sampling International.  They conduct opinion polls and market research for a variety of clients. Their calls do not lead to sales pitches nor are they an attempt to qualify you as a sales prospect for a third party so they are not required to screen against the DNC list before calling. Unfortunately, you do not seem to be able to opt out of calls while speaking with the poll/survey taker and have to request an opt out via their website.

https://www.surveysampling.com/general/received-a-call

They have numerous call centers in several states and foreign countries so there are multiple SSI number you may receive a call from.

This call comes it at least 3 times a day.  I have ask them to take our number off their list, but continue to get calls. Some of these calls are as late as 9PM.

Keep receiving calls as late as 9:30 PM.  When I get annoyed and answer there is no response.

This number shows up on my caller ID at least five times daily. No messages left, and I did actually pick up the phone last night, but the caller hung up immediately.