720-823-7535

Left no message

Left no voicemail

The person before already posted that, so what good did it do to post it again? None!  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

Apparently, this:

https://www.csffa-csfff.org/telemarketing

Might want to read it, as it is a "for profit" outfit:

Quote:

Telemarketing Campaign
The Colorado State Fire Fighters Foundation (CSFFF) is performing a telemarketing campaign to raise funds for downed firefighters and families, future firefighter training and education scholarships, and to support training at the annual CSFFA Fire College.

These phone calls are from the PB Entertainment group based in Memphis Tennessee. The callers are paid solicitors for the CSFFF and the foundation receives 27% of these donations.

They keep calling me but I have no idea who it is an won't pick up unless they leave a message

Called and then hung up on me when I answered.

I'm on the do not call registry - why are they calling?

The National Do Not Call Registry (DNC) does not cover calls requesting charitable donations. Charitable organizations and political campaigns are exempt from the DNC's rules.
However, For-profit organizations that solicit donations must honor requests to be removed from their entity with a specific internal do not call list.
Their websites:
https://csfff.org/telemarketing/
https://new.csfff.org/telemarketing/