Got 4 calls in a week and hung up

They call once a day Monday through Friday, have been for around a month now. Never say anything, usually the other end is dead and they hang up or the call drops but today(12/7/15) there was some sort of sound like they were using a car phone and could hear them driving then they hung up.

I get called a couple of times a week and after hang up or not speaking , I then blocked the call. Next day and 5-6 times later,
They change the phone number by one digit and call again/. I block/ they digit change and call again

I get called from this number and it sounds like someone is there but they don't day anything and you can hear them hang up.

I called them back and they took me off the calling list. I did have to call like three times.