As far as I can tell, cuz I have a lot of missed calls from spam or unknown numbers, I've had 2 missed calls that I've returned (on a cell phone, so it was a direct call back) in which the person on the other end claimed to they did not call me. One was a kindly older woman who said she hadn't used her phone and I called her back immediately. The other person, who I also called back immediately, was busy in a text conversation with someone else. I told the guy at Google, who I was waiting for a return call from and thought it was them, and he was confused.
Has anybody else had this issue? I'm sure it was a spam call, but coming from other people's numbers is concerning.