Received call from this number when I called back got a busy signal and then a third time an error message. It takes a while the call to pick up each time.

Same deal, no answer upon callback

They call and wait for voice mail to pick up, but leave no mesage

robot caller - telemarketer or scam but they didn't talk long enough to determine for sure which one (or if both)
they'll hang up if you keep talking in another language instead of English

Live telemarketer selling Home security. I politely told them their call violated the do not call list.