Was the check made out to cash? Lol. Why send money to a solicitor over the phone? Just give directly to the organization and you won't end up paying a scammer. This is why people get calls all day - because there are people out there who will pay.
It's a spoofed (faked) number, usually in the recipients (your) area code. Calling the number back will not get you to the people who called, it will just call the number on your caller id (which is not who called you, if it's even in service). It is not illegal to spoof numbers yet so there is nothing anyone will check on.
Also, if these people really are non profit (and not just scammers) then they can call people who are on the do not call list until you ask to be taken off of their list when they call.
These guys call about 4 or 5 times a day and never leave a message. My husband donated to special Olympics a few years ago and about 3 months ago talked to theses guys and said quit calling if I talk to them they will not leave a message. They have been calling numerous times a day for months.