Phone rang, answered with 'hello, hello' got reply 'goodbye' from automated female voice and it hung up... will be watching for a pest call follow up.

A nice article found on this topic
http://www.callhunter.com/guides/robocaller-why-do-you-not-speak  

Called but didn't leave a msg so they're now blocked.

Called me this morning as well. At first it didn't sound like anyone was there. I waited a few seconds and then there was a beep followed by an automated voice that said "Good bye". Tried calling the number back and it doesn't connect. I don't think it is a valid number.

Phone rang at 9:30 am - no one was on the line when I answered.