Calls frequently and hangs up. Never leaves message.

I receive calls from this number almost every day.  They never leave a message, so now I just don't answer the call.

Calls all the time and never leaves a message. When you call back you're on hold for approximately 10 minutes and then you get disconnected.

I always answer this call because it's a guy from Nigeria who is trying to give me millions of dollars in gold bullion.  I keep asking him about the details, but he won't talk.