Telemarketers -- when the call went to voice-mail, their "dialing" computer
thought that I answered.  So, the voice-mail message was "hell-o, hell-o"
followed by background noises from their "boiler-room" (presumably full of
people waiting for that "dialing" computer to route the "live" respondent
(my voice-mail) to the next available employee.

If it is Citi Bank, I wish they would lose their license.  I have told them so often I don't want their product and still they call and call and call.

they called my cellphone

I keep getting calls from this number and it's always a random person just talking, as if already in a conversation. Or sometimes they just say "Hello? Hello?" even though I am answering them,

got this call as well ( (416) 850-9355) second time actually, they didn't say anything. for those of you who called them back and chose the remove number option, did it work?

What is so important that they have the reapeatedly call and never leave a message.  And telemarketers wonder why some people screen their calls.  Duh, would they like us to call them this much?