Depending on your location, the caller could be using a number that is more likely to be answered than their real one. It's very possible the call's coming from outside the country, especially if they're coming at unreasonable hours. Don't rule out anything when it comes to telepests and crooks, and foreigners in particular.
I've noticed the same as Dave has and have blocked each number they call from.
They always call when I'm out and never leave a message.
Also, just city & state in the caller ID has ALWAYS been scammer calls for me and I always block them.
And when you do a search on the number you never find a legit company or person associated with it - another positive sign for scammers.