Telemarketer. I redialed and got the answering machine asking if I want to opt out. I am blocking # because these telemarketers often do not remove you but put your # on a "live" list and sell it.
Received a call, left no message. I called the number back from a different number and it rang about 10 times before going to a fast busy signal. I'm blocking them now.
Like others, I got a call, they left no message. Telemarketer, clearly. Do not answer or return such calls. Gary ^ was right. They are phishing for live numbers, which they then sell. Don't give them yours.