My case is a little odd...so, I'd gotten calls from this number on my cell phone. Numerous calls--at least one, but sometimes two a day, sometimes at night. I'd gotten a couple of voicemails that also sounded like they were butt dials--just background noise. I blocked the number on my phone and didn't get bothered for months...until the past couple weeks ago. This time, they were coming through as "Unknown Caller." I didn't know this until I downloaded a caller ID/call blocking app called "Hiya," then went down my call log to see which numbers I'd previously blocked. As it turned out, this was the only phone number I'd blocked in a while, so this was the only number I blacklisted in "Hiya." Well, wouldn't you know it...the spam calls from "Unknown Caller" stopped.
Moral of the story: Telemarketers and scammers just might try to keep harassing you as an "Unknown Caller" if they know you already blocked them on your phone. Having a second call blocking app just might help keep them from bothering you again.