This number has been calling me three or four times a day for several months.  It rings four times and then hangs up.  If I pick up the phone (and do not say anything), there are a couple of seconds of silence and then the line hangs up and a busy tone begins.  This "caller" seems to be some kind of monitoring or information gathering system.

Chronic (17 recent) hang-up phone calls from this number.

Also no message, calls frequently, sometime 3x per day.

No message left either