Annoying calls / fake calls / telemarketer calls / do not call list

Hello! We have been getting calls to our main line (no extension) but they do not leave a message, or leave a few seconds of blank message. They are not clients or leads of ours and appear from area codes across the US. Has anyone else had this issue? Are these likely automated telemarketing calls? Is there a do not call list I can enter? Thanks! Our number has been posted publicly on our site for a few years. This has been a problem for the past year. I realize five calls a day might not be a nuisance for some, but we receive a small call volume and our Director has stopped answering calls altogether because most of them are "fake".