What does it mean when caller id only displays three digits and nothing else?

I think it’s the area code but it could be a country code as well. It wasn’t a full number that displayed but only 502. Nothing else. I’ve had calls that said number unavailable or private but this is new to me.
I didn’t answer it, they didn’t leave a message and I did put the number in my reject list because I thought it might be a scam or something.
Has this happened to anyone else?