Telemarketers. If you hear silence, it means a computer was calling you. When you say hello, it will automatically switch over to a live person. It works most of the time but not all the time. That will explain the silence you heard.

I dont answer any call that starts with 800 or 866, but they do get saved with the silent ringer.

Just got a call from this number. No one answered. Not sure who is it, nothing on the ID.