I answer they hang up.

Didn't answer ....  no msg .... CID said INVALID NUMBER  ....  REALLY????

"Invalid Number" = spoofed.  For someone who knows what they are doing its not hard to make Caller ID say anything you want it to say.  In this case the caller made it say a number that is not in use.  Your caller was a telemarketer, a scammer or a pollster

All same as above