Called a few times in the past couple of days; they have not left a message on voicemail. I looked a few things up on the internet out of curiosity, and this seems to be a number used to solicit donations for the Illinois Special Olympics. Putting "ilso" in google returns one of their websites (soill.org) as topmatch - not listed in the google results at all, but also registered to them (and hosting a mirror of the site), is ilso.org.

It's a Lake Forest number, but I don't see it on their site, so it's likely a contracted call center for fundraising purposes.

I just wish they wouldn't call me at 8:30 at night.