This number has called me and I want to know whom it is. I tried calling it back however it is saying it is not a working number?

Called my phone several times and each time did not leave any message

This number has been calling daily to my mobile phone. If I pick up the phone and say, "hello," there is silence for a few seconds, at which time I hang up. I always assume these are either robocalls or someone who's soliciting for something.