866-841-8679

Who is this?

I would guess it is an unknown caller.
Missed calls could be from anyone.
Scammers, telemarketers, debt collectors, pranksters, surveys, notifications, alerts, wrong number, etc. Some use neighbor spoofing so it appears that an incoming call is coming from a local number, or spoof a number from a company or a government agency that you may already know and trust. If you answer, you can find out who it is or what they want. Just be aware some may use scam scripts to try to steal your money or valuable personal information, which can be used in fraudulent activity.
https://www.fcc.gov/spoofing

Called 4 times from 2 different numbers over the space of 24 hours. Asked for a person not associated with my cell phone, claimed they were interested in insurance quotes. Refused to tell me the name of the company, only referring to themselves as "insurance quotes".

I made the mistake of entering my information for an auto insurance quote and got called from this number within three seconds.  They haven't stopped...