I called the number back - it's from a fundraising group called Strategic Fundraising. There message is as follows:
"Hello, you've reached Strategic Fundraising. Strategic provides telefundraising services for political and nonprofit organizations throut the US. If you are wondering why you received a call from us, it is likely that we are calling you on behalf of a charity or a political party you support."
There are prompts to remove your number from their list.
I've had calls from this number - usually during the day, but no message is ever left. I've also received similar calls from Washington state and Arizona. I don't answer because I don't recognize the number (IL) and I don't know anyone in WA and AZ.