Part of a so-called "Missed call scam". The criminals hope you call back and will direct you to a premium-rate service which can cost you a lot. They do not say anything on the voicemails they leave on your phone because they want to tempt you to call them back.
Call 10 times a day. No one is speaking. No message. Very annoying. That kind of practice should be prohibited.
I have blocked this number and tomorrow It will probably be another number... Very annoying