Keep calling many times even though there is no answer

The caller must have got my mobile number through unauthorized means.

They may have also used a robo-dialer to do something called 'Sequential Dialing' where they dial every number in a section. First they would dial say 111-111-1111, then 111-111-1112 and so on. Whether or not your number is unlisted wouldn't mean anything if they went this route.