Just got a call from this number and ID says "Unknown" so if you are unknown to my number, I don't answer. They left now message!

Called landline - no message left.

has called several times. will not say anything when I answer. don't answer any more

Have many calls from this number - quit trying to rush to get the phone as they never leave a message and am not calling back to a long distance number that shows out of area.. Nuisance!

It's a robocall that sounds like a human.  You get "hello" but if you say nothing it will hang up.  If you say "hello" it will trigger the message, "Hi, thanks for calling back.  This is Joe Collins with the Texas state Fraternal Order of Police."  So it's a solicitation call.  There are warnings about the so-called FOP and other police and fireman charity calls.  Read here:
https://www.consumer.ftc.gov/articles/0085-fu ... nd-firefighters

You can check up on any charity here:
http://www.charitynavigator.org/
https://www.consumeraffairs.com/charity-scams