Receive a call several times a day, no one replies, then the call disconnects. Caller ID shows unknown name. I have called back and a recorded message allows you to add your number to the Do Not Call List, problem solved.
Try this: Hold your finger down on the number for a few seconds. If a message pops up 'block number' you can say yes. There are some every day cell phones that will allow you to do that.
When I get a call I don't know I don't answer and can hit on the number, then hit "more' instead of "delete" and add to auto reject list. I have a Samsung Android Galaxy. Check out with your provided how to do this. Every time you do this, it stops at least that number from ringing through again. I add to this list almost every day.